Nanostructured Battery Active Materials and Methods of Producing Same

ABSTRACT

Methods for producing nanostructures from copper-based catalysts on porous substrates, particularly silicon nanowires on carbon-based substrates for use as battery active materials, are provided. Related compositions are also described. In addition, novel methods for production of copper-based catalyst particles are provided. Methods for producing nanostructures from catalyst particles that comprise a gold shell and a core that does not include gold are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/511,826, filed Jul. 26, 2011, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The invention pertains to the field of nanotechnology. Moreparticularly, the invention relates to methods for producingnanostructures from copper-based catalyst materials, particularlysilicon nanostructures on carbon-based substrates for use as batteryactive materials. The invention also relates to compositions includingsilicon nanowires on porous substrates, particularly carbon-basedsubstrates that can serve as battery active materials.

BACKGROUND OF THE INVENTION

Silicon nanowires are desirable materials for many applications in thesemiconductor industry, as well as in production of medical devices andhigh capacity lithium-ion batteries. Gold nanoparticles have beenextensively used to catalyze growth of silicon nanowires. However, thecost of gold becomes significant or even prohibitive for large scalesynthesis of silicon nanowires, and gold is not compatible with alldesired applications for the nanowires.

Methods for silicon nanostructure growth that reduce or even eliminatethe need for a gold catalyst are thus desirable. Among other aspects,the present invention provides such methods. A complete understanding ofthe invention will be obtained upon review of the following.

SUMMARY OF THE INVENTION

Methods for producing nanostructures from copper-based catalysts onporous substrates, particularly silicon nanowires on carbon-basedsubstrates for use as battery active materials, are provided.Compositions including such nanostructures are described. Novel methodsfor production of copper-based catalyst particles are also provided.

One general class of embodiments provides methods for producingnanostructures. In the methods, a porous substrate having catalystparticles disposed thereon is provided, and the nanostructures are grownfrom the catalyst particles. The catalyst particles comprise copper, acopper compound, and/or a copper alloy.

The substrate can comprise, e.g., a carbon-based substrate, a populationof particles, a population of graphite particles, a plurality of silicaparticles, a plurality of carbon sheets, carbon powder, natural and/orartificial graphite, graphene, graphene powder, carbon fibers, carbonnanostructures, carbon nanotubes, carbon black, a mesh, or a fabric. Inone class of embodiments, the substrate comprises a population ofgraphite particles and the nanostructures are silicon nanowires.

The catalyst particles can be of essentially any desired size but aretypically nanoparticles. For example, the catalyst particles optionallyhave an average diameter between about 5 nm and about 100 nm, e.g.,between about 20 nm and about 50 nm, e.g., between about 20 nm and about40 nm.

As noted above, the catalyst particles can comprise copper, a coppercompound, and/or a copper alloy. For example, the catalyst particles cancomprise copper oxide. In one class of embodiments, the catalystparticles comprise copper (I) oxide (Cu₂O), copper (II) oxide (CuO), ora combination thereof. In one class of embodiments, the catalystparticles comprise elemental (i.e., pure-phase) copper (Cu), copper (I)oxide (Cu₂O), copper (II) oxide (CuO), or a combination thereof Inanother class of embodiments, the catalyst particles comprise copperacetate, copper nitrate, or a copper complex comprising a chelatingagent (e.g., copper tartrate or copper EDTA).

Catalyst particles can be produced and disposed on the substrate byessentially any convenient techniques, including, but not limited to,colloidal synthesis followed by deposition, adsorption of copper ions orcomplexes, and electroless deposition. Thus, in one class ofembodiments, providing a porous substrate having catalyst particlesdisposed thereon comprises synthesizing colloidal nanoparticlescomprising copper and/or a copper compound and then depositing thenanoparticles on the substrate. The nanoparticles optionally compriseelemental copper (Cu), copper (I) oxide (Cu₂O), copper (II) oxide (CuO),or a combination thereof, and the substrate optionally comprises apopulation of graphite particles. In another class of embodiments,providing a porous substrate having catalyst particles disposed thereoncomprises synthesizing discrete particles on the substrate throughelectroless deposition of copper onto the substrate, by immersing thesubstrate in an electroless plating solution comprising copper ions(e.g., at most 10 millimolar copper ions) and a reducing agent (e.g.,formaldehyde). The plating solution is typically alkaline. The substrateoptionally comprises a population of graphite particles. In anotherclass of embodiments, providing a porous substrate having catalystparticles disposed thereon comprises immersing the porous substrate in asolution comprising copper ions and/or a copper complex, whereby thecopper ions and/or the copper complex are adsorbed on the surface of thesubstrate, thereby forming discrete nanoparticles on the surface of thesubstrate. The solution is typically an aqueous alkaline solution. Thesubstrate optionally comprises a population of graphite particles.

The methods can be used to synthesize essentially any desired type ofnanostructures, including, but not limited to, nanowires. The nanowirescan be of essentially any desired size. For example, the nanowires canhave an average diameter less than about 150 nm, e.g., between about 10nm and about 100 nm, e.g., between about 30 nm and about 50 nm.

The nanostructures can be produced from any suitable material,including, but not limited to, silicon. In embodiments in which thenanostructures comprise silicon, the nanostructures can comprise, e.g.,monocrystalline silicon, polycrystalline silicon, amorphous silicon, ora combination thereof. Thus, in one class of embodiments, thenanostructures comprise a monocrystalline core and a shell layer,wherein the shell layer comprises amorphous silicon, polycrystallinesilicon, or a combination thereof. In one aspect, the nanostructures aresilicon nanowires.

The nanostructures can be grown using essentially any convenienttechnique. For example, silicon nanowires can be grown via avapor-liquid-solid (VLS) or vapor-solid-solid (VSS) technique.

The methods can be employed for production of nanostructures for use inany of a variety of different applications. For example, thenanostructures and the substrate on which they were grown can beincorporated into a battery slurry, battery anode, and/or battery, e.g.,a lithium ion battery.

In one class of embodiments, the substrate comprises a population ofgraphite particles and the nanostructures comprise silicon nanowires,and silicon comprises between 2% and 20% of the total weight of thenanostructures and the graphite particles after nanostructure growth iscompleted.

Another general class of embodiments provides methods for producingsilicon nanowires. In the methods, colloidal nanoparticles comprisingcopper and/or a copper compound are synthesized and deposited on asubstrate, and the nanowires are grown from the nanoparticles.

The copper compound is optionally copper oxide. In one class ofembodiments, the nanoparticles comprise elemental copper (Cu), copper(I) oxide (Cu₂O), copper (II) oxide (CuO), or a combination thereof. Thesize of the nanoparticles can vary, for example, depending on thediameter desired for the resulting nanowires. For example, thenanoparticles optionally have an average diameter between about 5 nm andabout 100 nm, e.g., between about 10 nm and about 100 nm, between about20 nm and about 50 nm, or between about 20 nm and about 40 nm.

Essentially all of the features noted for the embodiments above apply tothese embodiments as well, as relevant; for example, with respect totype and composition of substrate (e.g., a population of graphiteparticles), nanostructure growth technique (e.g., VLS or VSS), type,composition, and size of the resulting nanostructures, ratio ofnanostructures to substrate (e.g., silicon to graphite) by weight,incorporation into a battery slurry, battery anode, or battery, and/orthe like.

Another general class of embodiments provides methods for producingnanoparticles by electroless deposition. In the methods, a substrate isprovided. Also provided is an electroless plating solution thatcomprises at most 10 millimolar copper ions (e.g., Cu²⁺ and/or Cu⁺). Thesubstrate is immersed in the plating solution, whereby the copper ionsfrom the plating solution form discrete nanoparticles comprising copperand/or a copper compound on the substrate, until the plating solution issubstantially completely depleted of copper ions.

Suitable substrates include planar substrates, silicon wafers, foils,and nonporous substrates, in addition to porous substrates such as thosedescribed above, e.g., a population of particles, e.g., a population ofgraphite particles.

The substrate is typically activated prior to its immersion in theelectroless plating solution. The substrate is optionally activated bysoaking it in a solution of a metal salt, e.g., PdCl₂ or AgNO₃. Graphitesubstrates, however, particularly graphite particles which have a highsurface area, are conveniently activated simply by heating them prior toimmersion in the plating solution. Thus, in one class of embodiments thesubstrate comprises a population of graphite particles, which areactivated by heating to 20° C. or more (preferably 40° C. or more) priorto immersion in the plating solution.

In embodiments in which the substrate comprises a population ofparticles, the methods can include filtering the plating solution torecover the substrate particles from the plating solution after theplating solution is substantially completely depleted of copper ions.

The plating solution can include a copper salt, e.g., a copper (II)salt, as the copper source. The plating solution can include, e.g., oneor more of Rochelle salt, EDTA, and N,N,N′,N′-tetrakis (2-hydroxypropyl)ethylene-diamine) as a chelating agent. The plating solution caninclude, e.g., formaldehyde or sodium hypophosphite as the reducingagents. In one exemplary class of embodiments, the plating solutioncomprises a copper (II) salt, Rochelle salt, and formaldehyde and has analkaline pH.

As noted, the resulting nanoparticles can include copper or a coppercompound (for example, copper oxide). In one class of embodiments, thenanoparticles comprise elemental copper (Cu), copper (I) oxide (Cu₂O),copper (II) oxide (CuO), or a combination thereof. The resultingnanoparticles optionally have an average diameter between about 5 nm andabout 100 nm, e.g., between about 10 nm and about 100 nm, between about20 nm and about 50 nm, or between about 20 nm and about 40 nm.

The resulting nanoparticles are optionally employed as catalystparticles for subsequent synthesis of other nanostructures, e.g.,nanowires. Thus, the methods can include, after the plating solution issubstantially completely depleted of copper ions, removing the substratefrom the plating solution and then growing nanostructures (e.g.,nanowires, e.g., silicon nanowires) from the nanoparticles on thesubstrate.

Essentially all of the features noted for the embodiments above apply tothese embodiments as well, as relevant; for example, with respect tonanostructure growth technique (e.g., VLS or VSS), type, composition,and size of the resulting nanostructures, ratio of nanostructures tosubstrate (e.g., silicon to graphite) by weight, incorporation into abattery slurry, battery anode, or battery, and/or the like.

The plating solution can be employed as a single use bath or as areusable bath. Thus, in one class of embodiments, after the platingsolution is substantially completely depleted of copper ions, thesubstrate is removed from the plating solution, then copper ions areadded to the plating solution (e.g., by addition of a copper (II) salt),and then a second substrate is immersed in the plating solution.Typically, after addition of the copper ions, the plating solution againcomprises at most 10 millimolar copper ions. The second substrate istypically but need not be of the same type as the first substrate, e.g.,a second population of particles, e.g., graphite particles.

In embodiments in which the plating solution comprises formaldehyde,after the plating solution is substantially completely depleted ofcopper ions the formaldehyde can be treated by addition of sodiumsulfite to the plating solution prior to disposing of the solution.

As noted, nanoparticles produced by electroless deposition can beemployed as catalyst particles in subsequent nanostructure synthesisreactions. Accordingly, one general class of embodiments providesmethods for producing nanowires. In the methods, a substrate isprovided. An electroless plating solution comprising copper ions is alsoprovided, and the substrate is immersed in the plating solution, wherebythe copper ions from the plating solution form discrete nanoparticlescomprising copper and/or a copper compound on the substrate. Nanowiresare then grown from the nanoparticles on the substrate.

Essentially all of the features noted for the embodiments above apply tothese embodiments as well, as relevant; for example, with respect totype and composition of substrate (nonporous, porous, particles,graphite particles, sheets, wafers, etc.), activation of the substrate,size, shape, and composition of the nanoparticles (e.g., elementalcopper and/or copper oxide), components of the plating solution (coppersource and reducing, chelating, and other reagents), filtration step torecover a particulate substrate, reuse versus single use of the platingsolution, nanostructure growth technique (e.g., VLS or VSS), type,composition, and size of the resulting nanostructures, ratio ofnanostructures to substrate (e.g., silicon to graphite) by weight,incorporation into a battery slurry, battery anode, or battery, and/orthe like.

Nanoparticles produced by adsorption can be employed as catalystparticles in subsequent nanostructure synthesis reactions. Accordingly,another general class of embodiments provides methods for producingsilicon nanowires. In the methods, a substrate is provided. A solutioncomprising copper ions and/or a copper complex is also provided, and thesubstrate is immersed in the solution, whereby the copper ions and/orthe copper complex are adsorbed on the surface of the substrate, therebyforming discrete nanoparticles comprising a copper compound on thesurface of the substrate. The nanowires are then grown from thenanoparticles on the substrate.

The solution optionally includes a copper (II) salt (e.g., coppersulfate, copper acetate, or copper nitrate) and/or a copper complexcomprising a chelating agent (e.g., copper (II) tartrate or copperEDTA). The solution can be an aqueous solution, typically, an alkalinesolution.

The size of the nanoparticles can vary, for example, depending on thediameter desired for the resulting nanowires. For example, thenanoparticles optionally have an average diameter between about 5 nm andabout 100 nm.

Essentially all of the features noted for the embodiments above apply tothese embodiments as well, as relevant; for example, with respect totype and composition of substrate (e.g., a population of graphiteparticles), nanostructure growth technique (e.g., VLS or VSS),composition and size of the resulting nanowires, ratio of nanowires tosubstrate (e.g., silicon to graphite) by weight, incorporation into abattery slurry, battery anode, or battery, and/or the like.

Compositions produced by or useful in practicing any of the methodsherein are also a feature of the invention. Accordingly, one generalclass of embodiments provides a composition that includes a poroussubstrate and a population of silicon nanowires attached thereto,wherein one end of a member nanowire is attached to the substrate andthe other end of the member nanowire comprises copper, a coppercompound, and/or a copper alloy.

Essentially all of the features noted for the embodiments above apply tothese embodiments as well, as relevant; for example, with respect totype, composition, and size of nanostructures, composition andconfiguration of the substrate, catalyst material, incorporation into abattery slurry, battery anode, or battery, and/or the like.

For example, the composition can include nanowires having an averagediameter between about 10 nm and about 100 nm, e.g., between about 30 nmand about 50 nm, e.g., between about 40 nm and about 45 nm. Thenanowires can comprise monocrystalline silicon, polycrystalline silicon,amorphous silicon, or a combination thereof For example, the nanowiresoptionally comprise a monocrystalline core and a shell layer, whereinthe shell layer comprises amorphous silicon, polycrystalline silicon, ora combination thereof.

As for the embodiments above, the porous substrate is optionally acarbon-based substrate, a population of particles, a plurality of silicaparticles, a plurality of carbon sheets, carbon powder, natural and/orartificial graphite, a population of natural and/or artificial graphiteparticles, graphene, graphene powder, carbon fibers, carbonnanostructures, carbon nanotubes, carbon black, a mesh, or a fabric.

The catalyst-derived material on the ends of the member nanowires notattached to the substrate can comprise, e.g., elemental copper, copperoxide, copper silicide, or a combination thereof.

The composition optionally includes a polymer binder, e.g.,carboxymethyl cellulose. In one class of embodiments, the substratecomprises a population of graphite particles, and silicon comprisesbetween 2% and 20% of the total weight of the nanostructures and thegraphite particles.

A battery slurry, battery anode, or battery comprising the compositionis also a feature of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Panels A and B show scanning electron micrographs of colloidalCu₂O nanoparticles synthesized in an aqueous medium.

FIG. 2 Panel A shows a scanning electron micrograph of colloidal Cu₂Onanoparticles deposited on a particulate graphite substrate. Panel Bshows a scanning electron micrograph of silicon nanowires grown from theCu₂O nanoparticles on graphite particles.

FIG. 3 Panel A shows a scanning electron micrograph of coppernanoparticles deposited on a particulate graphite substrate from anelectroless plating solution. Panel B shows a scanning electronmicrograph of silicon nanowires grown from the copper nanoparticles.

FIG. 4 Panel A schematically illustrates VLS growth of a siliconnanowire from a gold catalyst particle. Panel B schematicallyillustrates VLS growth of a silicon nanowire from a non-gold core/goldshell catalyst particle. Panel C presents a graph showing the percentageof the nanoparticle volume occupied by the non-Au material (i.e., thevolume of the core as a percentage of the overall volume including boththe core and the shell) for a 15 nm non-Au core coated with an Au shellof varying thickness.

FIG. 5 Panel A shows scanning electron micrographs of nanoparticlesdeposited on a particulate graphite substrate by electroless deposition(row I) and by adsorption (row II), at increasing magnification fromleft to right. Panel B shows scanning electron micrographs of siliconnanowires grown from the nanoparticles produced by electrolessdeposition (row I) and adsorption (row II), at increasing magnificationfrom left to right.

Schematic figures are not necessarily to scale.

DEFINITIONS

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the invention pertains. The following definitionssupplement those in the art and are directed to the current applicationand are not to be imputed to any related or unrelated case, e.g., to anycommonly owned patent or application. Although any methods and materialssimilar or equivalent to those described herein can be used in thepractice for testing of the present invention, the preferred materialsand methods are described herein. Accordingly, the terminology usedherein is for the purpose of describing particular embodiments only, andis not intended to be limiting.

As used in this specification and the appended claims, the singularforms “a,” “an” and “the” include plural referents unless the contextclearly dictates otherwise. Thus, for example, reference to “ananostructure” includes a plurality of such nanostructures, and thelike.

The term “about” as used herein indicates the value of a given quantityvaries by +/−10% of the value, or optionally +/−5% of the value, or insome embodiments, by +/−1% of the value so described.

A “nanostructure” is a structure having at least one region orcharacteristic dimension with a dimension of less than about 500 nm,e.g., less than about 200 nm, less than about 100 nm, less than about 50nm, or even less than about 20 nm. Typically, the region orcharacteristic dimension will be along the smallest axis of thestructure. Examples of such structures include nanowires, nanorods,nanotubes, nanofibers, branched nanostructures, nanotetrapods, tripods,bipods, nanocrystals, nanodots, quantum dots, nanoparticles, and thelike. Nanostructures can be, e.g., substantially crystalline,substantially monocrystalline, polycrystalline, amorphous, or acombination thereof. In one aspect, each of the three dimensions of thenanostructure has a dimension of less than about 500 nm, e.g., less thanabout 200 nm, less than about 100 nm, less than about 50 nm, or evenless than about 20 nm.

An “aspect ratio” is the length of a first axis of a nanostructuredivided by the average of the lengths of the second and third axes ofthe nanostructure, where the second and third axes are the two axeswhose lengths are most nearly equal each other. For example, the aspectratio for a perfect rod would be the length of its long axis divided bythe diameter of a cross-section perpendicular to (normal to) the longaxis.

As used herein, the “diameter” of a nanostructure refers to the diameterof a cross-section normal to a first axis of the nanostructure, wherethe first axis has the greatest difference in length with respect to thesecond and third axes (the second and third axes are the two axes whoselengths most nearly equal each other). The first axis is not necessarilythe longest axis of the nanostructure; e.g., for a disk-shapednanostructure, the cross-section would be a substantially circularcross-section normal to the short longitudinal axis of the disk. Wherethe cross-section is not circular, the diameter is the average of themajor and minor axes of that cross-section. For an elongated or highaspect ratio nanostructure, such as a nanowire, the diameter is measuredacross a cross-section perpendicular to the longest axis of thenanowire. For a spherical nanostructure, the diameter is measured fromone side to the other through the center of the sphere.

The terms “crystalline” or “substantially crystalline,” when used withrespect to nanostructures, refer to the fact that the nanostructurestypically exhibit long-range ordering across one or more dimensions ofthe structure. It will be understood by one of skill in the art that theterm “long range ordering” will depend on the absolute size of thespecific nanostructures, as ordering for a single crystal cannot extendbeyond the boundaries of the crystal. In this case, “long-rangeordering” will mean substantial order across at least the majority ofthe dimension of the nanostructure. In some instances, a nanostructurecan bear an oxide or other coating, or can be comprised of a core and atleast one shell. In such instances it will be appreciated that theoxide, shell(s), or other coating need not exhibit such ordering (e.g.it can be amorphous, polycrystalline, or otherwise). In such instances,the phrase “crystalline,” “substantially crystalline,” “substantiallymonocrystalline,” or “monocrystalline” refers to the central core of thenanostructure (excluding the coating layers or shells). The terms“crystalline” or “substantially crystalline” as used herein are intendedto also encompass structures comprising various defects, stackingfaults, atomic substitutions, and the like, as long as the structureexhibits substantial long range ordering (e.g., order over at leastabout 80% of the length of at least one axis of the nanostructure or itscore). In addition, it will be appreciated that the interface between acore and the outside of a nanostructure or between a core and anadjacent shell or between a shell and a second adjacent shell maycontain non-crystalline regions and may even be amorphous. This does notprevent the nanostructure from being crystalline or substantiallycrystalline as defined herein.

The term “monocrystalline” when used with respect to a nanostructureindicates that the nanostructure is substantially crystalline andcomprises substantially a single crystal. When used with respect to ananostructure heterostructure comprising a core and one or more shells,“monocrystalline” indicates that the core is substantially crystallineand comprises substantially a single crystal.

A “nanocrystal” is a nanostructure that is substantiallymonocrystalline. A nanocrystal thus has at least one region orcharacteristic dimension with a dimension of less than about 500 nm,e.g., less than about 200 nm, less than about 100 nm, less than about 50nm, or even less than about 20 nm. The term “nanocrystal” is intended toencompass substantially monocrystalline nanostructures comprisingvarious defects, stacking faults, atomic substitutions, and the like, aswell as substantially monocrystalline nanostructures without suchdefects, faults, or substitutions. In the case of nanocrystalheterostructures comprising a core and one or more shells, the core ofthe nanocrystal is typically substantially monocrystalline, but theshell(s) need not be. In one aspect, each of the three dimensions of thenanocrystal has a dimension of less than about 500 nm, e.g., less thanabout 200 nm, less than about 100 nm, less than about 50 nm, or evenless than about 20 nm. Examples of nanocrystals include, but are notlimited to, substantially spherical nanocrystals, branched nanocrystals,and substantially monocrystalline nanowires, nanorods, nanodots, quantumdots, nanotetrapods, tripods, bipods, and branched tetrapods (e.g.,inorganic dendrimers).

The term “heterostructure” when used with reference to nanostructuresrefers to nanostructures characterized by at least two different and/ordistinguishable material types. Typically, one region of thenanostructure comprises a first material type, while a second region ofthe nanostructure comprises a second material type. In certainembodiments, the nanostructure comprises a core of a first material andat least one shell of a second (or third etc.) material, where thedifferent material types are distributed radially about the long axis ofa nanowire, a long axis of an arm of a branched nanowire, or the centerof a nanocrystal, for example. (A shell can but need not completelycover the adjacent materials to be considered a shell or for thenanostructure to be considered a heterostructure; for example, ananocrystal characterized by a core of one material covered with smallislands of a second material is a heterostructure.) In otherembodiments, the different material types are distributed at differentlocations within the nanostructure; e.g., along the major (long) axis ofa nanowire or along a long axis of arm of a branched nanowire. Differentregions within a heterostructure can comprise entirely differentmaterials, or the different regions can comprise a base material (e.g.,silicon) having different dopants or different concentrations of thesame dopant.

A “nanoparticle” is a nanostructure in which each dimension (e.g., eachof the nanostructure's three dimensions) is less than about 500 nm,e.g., less than about 200 nm, less than about 100 nm, less than about 50nm, or even less than about 20 nm. Nanoparticles can be of any shape,and include, for example, nanocrystals, substantially sphericalparticles (having an aspect ratio of about 0.8 to about 1.2), andirregularly shaped particles. Nanoparticles optionally have an aspectratio less than about 1.5. Nanoparticles can be amorphous, crystalline,monocrystalline, partially crystalline, polycrystalline, or otherwise.Nanoparticles can be substantially homogeneous in material properties,or in certain embodiments can be heterogeneous (e.g., heterostructures).Nanoparticles can be fabricated from essentially any convenient materialor materials, e.g., the nanoparticles can comprise “pure” materials,substantially pure materials, doped materials and the like.

A “nanowire” is a nanostructure that has one principle axis that islonger than the other two principle axes. Consequently, the nanowire hasan aspect ratio greater than one; nanowires of this invention typicallyhave an aspect ratio greater than about 1.5 or greater than about 2.Short nanowires, sometimes referred to as nanorods, typically have anaspect ratio between about 1.5 and about 10. Longer nanowires have anaspect ratio greater than about 10, greater than about 20, greater thanabout 50, or greater than about 100, or even greater than about 10,000.The diameter of a nanowire is typically less than about 500 nm,preferably less than about 200 nm, more preferably less than about 150nm, and most preferably less than about 100 nm, about 50 nm, or about 25nm, or even less than about 10 nm or about 5 nm. The nanowires of thisinvention can be substantially homogeneous in material properties, or incertain embodiments can be heterogeneous (e.g., nanowireheterostructures). The nanowires can be fabricated from essentially anyconvenient material or materials. The nanowires can comprise “pure”materials, substantially pure materials, doped materials and the like,and can include insulators, conductors, and semiconductors. Nanowiresare typically substantially crystalline and/or substantiallymonocrystalline, but can be, e.g., polycrystalline or amorphous. In someinstances, a nanowire can bear an oxide or other coating, or can becomprised of a core and at least one shell. In such instances it will beappreciated that the oxide, shell(s), or other coating need not exhibitsuch ordering (e.g. it can be amorphous, polycrystalline, or otherwise).Nanowires can have a variable diameter or can have a substantiallyuniform diameter, that is, a diameter that shows a variance less thanabout 20% (e.g., less than about 10%, less than about 5%, or less thanabout 1%) over the region of greatest variability and over a lineardimension of at least 5 nm (e.g., at least 10 nm, at least 20 nm, or atleast 50 nm). Typically the diameter is evaluated away from the ends ofthe nanowire (e.g., over the central 20%, 40%, 50%, or 80% of thenanowire). A nanowire can be straight or can be, e.g., curved or bent,over the entire length of its long axis or a portion thereof. In certainembodiments, a nanowire or a portion thereof can exhibit two- orthree-dimensional quantum confinement. Nanowires according to thisinvention can expressly exclude carbon nanotubes, and, in certainembodiments, exclude “whiskers” or “nanowhiskers”, particularly whiskershaving a diameter greater than 100 nm, or greater than about 200 nm.

A “substantially spherical nanoparticle” is a nanoparticle with anaspect ratio between about 0.8 and about 1.2. Similarly, a“substantially spherical nanocrystal” is a nanocrystal with an aspectratio between about 0.8 and about 1.2.

A “catalyst particle” or “nanostructure catalyst” is a material thatfacilitates the formation or growth of a nanostructure. The term is usedherein as it is commonly used in the art relevant to nanostructuregrowth; thus, use of the word “catalyst” does not necessarily imply thatthe chemical composition of the catalyst particle as initially suppliedin a nanostructure growth reaction is identical to that involved in theactive growth process of the nanostructure and/or recovered when growthis halted. For example, when gold nanoparticles are used as catalystparticles for silicon nanowire growth, particles of elemental gold aredisposed on a substrate and elemental gold is present at the tip of thenanowire after synthesis, though during synthesis the gold exists as aeutectic phase with silicon. As a contrasting example, withoutlimitation to any particular mechanism, when copper nanoparticles areused for VLS or VSS nanowire growth, particles of elemental copper aredisposed on a substrate, and copper silicide may be present at the tipof the nanowire during and after synthesis. As yet another example,again without limitation to any particular mechanism, when copper oxidenanoparticles are used as catalyst particles for silicon nanowiregrowth, copper oxide particles are disposed on a substrate, but they maybe reduced to elemental copper in a reducing atmosphere employed fornanowire growth and copper silicide may be present at the tip of thenanowire during and after nanowire synthesis. Both situations in whichthe catalyst material maintains the identical chemical composition andsituations in which the catalyst material changes in chemicalcomposition are explicitly included by used of the terms “catalystparticle” or “nanostructure catalyst” herein. Catalyst particles aretypically nanoparticles, particularly discrete nanoparticles. Catalystparticles are distinct from precursors employed during nanostructuregrowth, in that precursors, in contrast to the catalyst particles, serveas a source for at least one type of atom that is incorporatedthroughout the nanostructure (or throughout a core, shell, or otherregion of a nanostructure heterostructure).

A “compound” or “chemical compound” is a chemical substance consistingof two or more different chemical elements and having a unique anddefined chemical structure, including, e.g., molecular compounds heldtogether by covalent bonds, salts held together by ionic bonds,intermetallic compounds held together by metallic bonds, and complexesheld together by coordinate covalent bonds.

An “alloy” is a metallic solid solution (complete or partial) composedof two or more elements. A complete solid solution alloy has a singlesolid phase microstructure, while a partial solution alloy has two ormore phases that may or may not be homogeneous in distribution.

A “porous” substrate contains pores or voids. In certain embodiments, aporous substrate can be an array or population of particles, e.g., arandom close pack particle population or a dispersed particlepopulation. The particles can be of essentially any desired size and/orshape, e.g., spherical, elongated, oval/oblong, plate-like (e.g.,plates, flakes, or sheets), or the like. The individual particles canthemselves be nonporous or can be porous (e.g., include a capillarynetwork through their structure). When employed for nanostructuregrowth, the particles can be but typically are not cross-linked. Inother embodiments, a porous substrate can be a mesh or fabric.

A “carbon-based substrate” refers to a substrate that comprises at leastabout 50% carbon by mass. Suitably, a carbon-based substrate comprisesat least about 60% carbon, 70% carbon, 80% carbon, 90% carbon, 95%carbon, or about 100% carbon by mass, including 100% carbon. Exemplarycarbon-based substrates that can be used in the practice of the presentinvention include, but are not limited to, carbon powder, such as carbonblack, fullerene soot, desulfurized carbon black, graphite, graphitepowder, graphene, graphene powder, or graphite foil. As used throughout,“carbon black” refers to the material produced by the incompletecombustion of petroleum products. Carbon black is a form of amorphouscarbon that has an extremely high surface area to volume ratio.“Graphene” refers to a single atomic layer of carbon formed as a sheet,and can be prepared as graphene powders. See, e.g., U.S. Pat. Nos.5,677,082, 6,303,266 and 6,479,030, the disclosures of each of which areincorporated by reference herein in their entireties. Carbon-basedsubstrates specifically exclude metallic materials, such as steel,including stainless steel. Carbon-based substrates can be in the form ofsheets or separate particles, as well as cross-linked structures.

Unless clearly indicated otherwise, ranges listed herein are inclusive.

A variety of additional terms are defined or otherwise characterizedherein.

DETAILED DESCRIPTION

Traditional batteries, including lithium ion batteries, comprise ananode, an electrolyte, a cathode, and typically a separator. The anodeof most commercially available lithium ion batteries is copper foilcoated with a mixture of graphite powder and a polymer blend. Thecapacity of these materials is limited, however. There is therefore needfor improved anode materials with greater storage capacity.

Silicon has a high theoretical specific capacity for lithium (Li)storage (approximately 4200 mAh/g). However, silicon also experiences alarge volume change on lithiation or delithiation that renders bulksilicon impractical for use in battery active materials. Incorporationof silicon nanowires into anodes can minimize the mechanical stressassociated with lithium ion insertion and extraction. The use of siliconnanowires in anodes also provides very high silicon surface area andthus high charging rates. For additional information on incorporation ofsilicon into battery anodes, see, e.g., U.S. patent applicationpublication no. 2010/0297502 by Zhu et al. entitled “Nanostructuredmaterials for battery applications” and references therein, each ofwhich is incorporated by reference herein in its entirety. Chen et al.(2011) “Hybrid silicon-carbon nanostructured composites as superioranodes for lithium ion batteries” Nano Res. 4(3):290-296, Cui et al.(2009) “Carbon-silicon core-shell nanowires as high capacity electrodefor lithium ion batteries” Nano Letters 9(9)3370-3374, Chen et al.(2010) “Silicon nanowires with and without carbon coating as anodematerials for lithium-ion batteries” J Solid State Electrochem14:1829-1834, and Chan et al. (2010) “Solution-grown silicon nanowiresfor lithium-ion battery anodes” ACS Nano 4(3):1443-1450.

Widespread adoption of lithium ion batteries including siliconnanowire-based anodes, however, requires large scale synthesis ofsilicon nanowires. Currently, silicon nanowires are typically grownusing gold catalyst particles, for example, in a vapor-liquid-solid(VLS), chemical vapor deposition (CVD) process in which a feed gas(e.g., silane) is used as the source material. Gold catalyst on a heatedsolid substrate is exposed to the feed gas, liquifies, and absorbs theSi vapor to supersaturation levels. Nanostructure growth occurs at theliquid-solid interface. See, e.g., U.S. Pat. No 7,301,199 to Lieber etal. entitled “Nanoscale wires and related devices,” U.S. Pat. No.7,211,464 to Lieber et al. entitled “Doped elongated semiconductors,growing such semiconductors, devices including such semiconductors andfabricating such devices,” Cui et al. (2001) “Diameter-controlledsynthesis of single-crystal silicon nanowires” Appl. Phys. Lett. 78,2214-2216, and Morales et al. (1998) “A laser ablation method for thesynthesis of crystalline semiconductor nanowires” Science 279, 208-211.

However, the cost of gold becomes significant when large scale synthesisof silicon nanowires is contemplated. Additionally, the liquid state ofAu—Si at the eutectic temperature can cause uncontrollable deposition ofgold-based catalyst material and subsequent silicon growth at undesiredlocations such as on the substrate or the sidewalls of the nanowires.Furthermore, gold is not compatible with semiconductor processing and isprohibited in industrial clean rooms, which raises additionaldifficulties for gold catalyzed synthesis of nanowires intended for suchapplications.

In one aspect, the present invention overcomes the above noteddifficulties by providing methods of producing nanostructures (includingsilicon nanowires) that reduce or even eliminate need for a goldcatalyst. For example, methods for growing silicon nanowires fromcore-shell nanoparticles having a gold shell are provided that reducethe amount of gold required for nanostructure synthesis, as compared totraditional synthesis techniques using solid gold nanoparticlecatalysts. As another example, methods for growing silicon nanowires andother nanostructures from copper-based catalysts are provided thateliminate any need for a gold catalyst. The methods optionally includegrowing the nanostructures on a carbon-based porous substrate suitablefor incorporation into a battery anode. Compositions, battery slurries,battery anodes, and batteries including nanostructures grown on suchsubstrates from copper-based catalysts are also described. In addition,methods for production of nanoparticles including copper or a coppercompound and suitable for use as nanostructure catalysts are provided.

Nanostructure Growth Using Copper-Based Catalyst Materials

Although growth of silicon nanowires from copper-based catalysts hasbeen described in U.S. Pat. No. 7,825,036 to Yao et al. entitled “Methodof synthesizing silicon wires” and Renard et al. (2010) “Catalystpreparation for CMOS-compatible silicon nanowire synthesis” NatureNanotech 4:654-657, these methods produce nanowires on planar solidsubstrates not suitable for use as a battery active material or amenableto scaling up for production of large quantities of nanowires. Incontrast, one aspect of the present invention provides methods forgrowth of nanostructures (including but not limited to siliconnanowires) on porous or particulate substrates, including substratesthat are suitable for use in batteries and/or that facilitatelarge-scale nanostructure synthesis.

Thus, one general class of embodiments provides methods for producingnanostructures. In the methods, a porous substrate having catalystparticles disposed thereon is provided, and the nanostructures are grownfrom the catalyst particles. The catalyst particles comprise copper, acopper compound, and/or a copper alloy.

The porous substrate is optionally a mesh, fabric, e.g., a woven fabric(e.g., a carbon fabric), or fibrous mat. In preferred embodiments, thesubstrate comprises a population of particles, sheets, fibers(including, e.g., nanofibers), and/or the like. Thus, exemplarysubstrates include a plurality of silica particles (e.g., a silicapowder), a plurality of carbon sheets, carbon powder (a plurality ofcarbon particles), natural and/or artificial (synthetic) graphite,natural and/or artificial (synthetic) graphite particles, graphene,graphene powder (a plurality of graphene particles), carbon fibers,carbon nanostructures, carbon nanotubes, and carbon black. For synthesisof nanostructures, e.g., silicon nanowires, for use as a battery activematerial, the substrate is typically a carbon-based substrate, forexample, a population of graphite particles. Suitable graphite particlesare commercially available, for example, from Hitachi Chemical Co., Ltd.(Ibaraki, Japan, e.g., MAG D-13 artificial graphite).

In embodiments in which the substrate comprises a population ofparticles (e.g., graphite particles), the particles can be ofessentially any desired shape, for example, spherical or substantiallyspherical, elongated, oval/oblong, plate-like (e.g., plates, flakes, orsheets), and/or the like. Similarly, the substrate particles (e.g.,graphite particles) can be of essentially any size. Optionally, thesubstrate particles have an average diameter between about 0.5 μm andabout 50 μm, e.g., between about 0.5 μm and about 2 μm, between about 2μm and about 10 μm, between about 2 μm and about 5 μm, between about 5μm and about 50 μm, between about 10 μm and about 30 μm, between about10 μm and about 20 μm, between about 15 μm and about 25 μm, betweenabout 15 μm and about 20 μm, or about 20 μm. As will be evident, thesize of the substrate particles can be influenced by the applicationultimately desired for the resulting nanostructures. For example, wheresilicon nanostructures (e.g., silicon nanowires) are being synthesizedon a population of graphite particles as the substrate, the graphiteparticle size is optionally about 10-20 μm (e.g., about 15-20 μm) wherethe graphite particles and silicon nanostructures are to be incorporatedinto a battery where high storage capacity is desired, whereas graphiteparticle size is optionally a few μm (e.g., about 5 μm or less) wherethe graphite particles and silicon nanostructures are to be incorporatedinto a battery capable of delivering high current or power. For thelatter application, spherical graphite particles are optionally employedto achieve higher particle density.

The catalyst particles are disposed on the surface of the substrate.Thus, for example, where the substrate comprises a population ofparticles, the catalyst particles are disposed on the surface ofindividual substrate particles. Individual substrate particles canthemselves be porous or nonporous. Where porous particles are employedas the substrate, the catalyst particles are typically disposed on theouter surface of the substrate particles, but can additionally oralternatively be disposed on the interior surface of micropores orchannels within the substrate particles.

The catalyst particles can be of essentially any shape, including, butnot limited to, spherical or substantially spherical, plate-like,oval/oblong, cubic, and/or irregular shapes (e.g., starfish-shaped).Similarly, the catalyst particles can be of essentially any desired sizebut are typically nanoparticles. For example, the catalyst particlesoptionally have an average diameter between about 5 nm and about 100 nm,e.g., between about 10 nm and about 100 nm, between about 20 nm andabout 50 nm, or between about 20 nm and about 40 nm. Optionally, thecatalyst particles have an average diameter of about 20 nm. As is knownin the art, the size of the catalyst particles affects the size of theresulting nanostructures (e.g., the diameter of resulting nanowires).

As noted above, the catalyst particles can comprise copper, a coppercompound, and/or a copper alloy. For example, the catalyst particles cancomprise copper oxide, e.g., copper (I) oxide (cuprous oxide, Cu₂O),copper (II) oxide (cupric oxide, CuO), Cu₂O₃, Cu₃O₄, or a combinationthereof. Thus, in one class of embodiments, the catalyst particlescomprise copper (I) oxide (Cu₂O), copper (II) oxide (CuO), or acombination thereof. In one class of embodiments, the catalyst particlescomprise elemental (i.e., pure-phase) copper (Cu), copper (I) oxide(Cu₂O), copper (II) oxide (CuO), or a combination thereof. In one classof embodiments, the catalyst particles comprise elemental copper and aresubstantially free of copper compounds (e.g., copper oxide) or alloys,e.g., as determined by x-ray diffraction (XRD) and/or energy-dispersiveX-ray spectroscopy (EDS). In another class of embodiments, the catalystparticles consist essentially of copper oxide (e.g., Cu₂O and/or CuO),e.g., as determined by XRD and/or EDS. In another class of embodiments,the catalyst particles comprise copper acetate, copper nitrate, or acopper complex comprising a chelating agent (e.g., copper tartrate orcopper EDTA), preferably a copper (II) compound or complex. In one classof embodiments, the catalyst particles comprise a Cu—Ni alloy.

As noted above, the chemical composition of the catalyst particle asinitially supplied in a nanostructure growth reaction may not beidentical to that involved in the active growth process of thenanostructure and/or recovered when growth is halted. For example, whencopper oxide nanoparticles are used as catalyst particles for siliconnanowire growth, copper oxide particles are disposed on a substrate, butthey may be reduced to elemental copper in a reducing atmosphereemployed for VSS nanowire growth and copper silicide may be present atthe tip of the nanowire during and after such synthesis. As anotherexample, when elemental copper nanoparticles are used as catalystparticles for silicon nanowire growth, copper particles are disposed ona substrate, but they may be oxidized to copper oxide in ambientatmosphere, then reduced to elemental copper in a reducing atmosphereemployed for VSS nanowire growth, and copper silicide may be present atthe tip of the nanowire during and after such synthesis. As yet anotherexample, when nanoparticles comprising a copper compound such as copperacetate, copper nitrate, or a copper complex including a chelating agentare used as catalyst particles for silicon nanowire growth, they maydecompose to form copper oxide when heated in ambient atmosphere andthen be reduced to elemental copper in a reducing atmosphere employedfor nanowire growth, and copper silicide may be present at the tip ofthe nanowire during and after such synthesis.

Catalyst particles can be produced and disposed on the substrate byessentially any convenient techniques, including, but not limited to,colloidal synthesis followed by deposition, adsorption of copper ions orcomplexes, or electroless deposition. Thus, in one class of embodiments,providing a porous substrate having catalyst particles disposed thereoncomprises synthesizing colloidal nanoparticles comprising copper and/ora copper compound and then depositing the nanoparticles on thesubstrate. For additional details on colloidal synthesis of copper-basednanoparticles, see the section entitled “Colloidal Synthesis ofCopper-Based Nanoparticles” hereinbelow. In another class ofembodiments, providing a porous substrate having catalyst particlesdisposed thereon comprises synthesizing discrete particles on thesubstrate through electroless deposition of copper directly onto thesubstrate. For additional details on electroless deposition ofcopper-based nanoparticles, see the section entitled “ElectrolessDeposition of Copper-Based Nanoparticles” hereinbelow. In another classof embodiments, providing a porous substrate having catalyst particlesdisposed thereon comprises immersing the porous substrate in a solutioncomprising copper ions and/or a copper complex, whereby the copper ionsand/or the copper complex are adsorbed on the surface of the substrate,thereby forming discrete nanoparticles on the surface of the substrate.For additional details on production of copper-based nanoparticles viaadsorption, see the section entitled “Formation of Copper-BasedNanoparticles Through Adsorption” hereinbelow.

The methods can be used to synthesize essentially any desired type ofnanostructures, including, but not limited to, nanowires, whiskers ornanowhiskers, nanofibers, nanotubes, tapered nanowires or spikes,nanodots, nanocrystals, branched nanostructures having three or morearms (e.g., nanotetrapods), or a combination of any of these.

The nanostructures can be produced from any suitable material, suitablyan inorganic material, and more suitably an inorganic conductive orsemiconductive material. Suitable semiconductor materials include, e.g.,group II-VI, group III-V, group IV-VI, and group IV semiconductors.Suitable semiconductor materials include, but are not limited to, Si,Ge, Sn, Se, Te, B, C (including diamond), P, BN, BP, BAs, AlN, AlP,AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, AlN, AlP, AlAs,AlSb, GaN, GaP, GaAs, GaSb, ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS,HgSe, HgTe, BeS, BeSe, BeTe, MgS, MgSe, GeS, GeSe, GeTe, SnS, SnSe,SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr, CuI, Si₃N₄, Ge₃N₄, Al₂O₃,(Al, Ga, In)₂ (S, Se, Te)₃, Al₂CO₃ and an appropriate combination of twoor more such semiconductors.

In one aspect, for example, where the resulting nanostructures are to beincorporated into a lithium ion battery, the nanostructures comprisegermanium, silicon, or a combination thereof. In embodiments in whichthe nanostructures comprise silicon, the nanostructures can comprise,e.g., monocrystalline silicon, polycrystalline silicon, amorphoussilicon, or a combination thereof. For example, the nanostructures cancomprise about 20-100% monocrystalline silicon, about 0-50%polycrystalline silicon, and/or about 0-50% amorphous silicon. In oneclass of embodiments, the nanostructures comprise 20-100% (e.g.,50-100%) monocrystalline silicon and 0-50% amorphous silicon. In oneclass of embodiments, the nanostructures comprise 20-100% (e.g.,50-100%) monocrystalline silicon and 0-50% polycrystalline silicon. Thepercentage of monocrystalline, polycrystalline, and/or amorphous siliconcan be measured for the resulting nanostructures as a group orindividually. Individual silicon nanostructures (e.g., nanowires) can bea combination of crystalline, polycrystalline and amorphous material asdetected by transmission electron microscopy (TEM). For example,nanowires can be completely monocrystalline, can have a monocrystallinecore and a polycrystalline shell, can have a monocrystalline core and anamorphous or microcrystalline shell (where the grain structure is notvisible within the resolution of TEM), or can have a monocrystallinecore and a shell that transitions from polycrystalline to amorphous(from the core to the outside of the nanostructure). Thus, in one classof embodiments, the nanostructures comprise a monocrystalline core and ashell layer, wherein the shell layer comprises amorphous silicon,polycrystalline silicon, or a combination thereof.

The nanostructures optionally include a coating. For example, siliconnanostructures optionally bear a silicon oxide coating. As described inU.S. patent application publication no. 2010/0297502 by Zhu et al.entitled “Nanostructured materials for battery applications,” a carboncoating can be applied to the silicon nanostructures, e.g., where thenanostructures are intended for incorporation into a battery anode. Thenanostructures optionally have a polymer coating. See also, e.g., U.S.Pat. No. 7,842,432 to Niu et al. entitled “Nanowire structurescomprising carbon” and U.S. patent application publication no.2011/0008707 by Muraoka et al. entitled “Catalyst layer for fuel cellmembrane electrode assembly, fuel cell membrane electrode assembly usingthe catalyst layer, fuel cell, and method for producing the catalystlayer.”

In one aspect, the nanostructures are silicon nanowires. Nanowiresproduced by the methods can be of essentially any desired size. Forexample, the nanowires can have a diameter of about 10 nm to about 500nm, or about 20 nm to about 400 nm, about 20 nm to about 300 nm, about20 nm to about 200 nm, about 20 nm to about 100 nm, about 30 nm to about100 nm, or about 40 nm to about 100 nm. Typically, the nanowires have anaverage diameter less than about 150 nm, e.g., between about 10 nm andabout 100 nm, e.g., between about 30 nm and about 50 nm, e.g., betweenabout 40 nm and about 45 nm. The nanowires are optionally less thanabout 100 mm in length, e.g., less than about 10 μm, about 100 nm toabout 100 μm, or about 1 μm to about 75 μm, about 1 μm to about 50 μm,or about 1 mm to about 20 μm in length. The aspect ratios of thenanowires are optionally up to about 2000:1 or about 1000:1. Forexample, the nanowires can have a diameter of about 20 nm to about 200nm and a length of about 0.1 μm to about 50 mm.

The nanostructures can be synthesized using essentially any convenienttechnique. As one example, a vapor-liquid-solid (VLS) technique such asthat described above for gold catalyst particles can be employed withthe copper-based catalyst. VLS techniques employing copper catalyststypically require high temperatures, however (e.g., above 800° C. forsilicon nanowires). Vapor-solid-solid (VSS) techniques in which thecopper-based catalyst remains in the solid phase are typically moreconvenient since they can be performed at lower temperatures (e.g.,about 500° C. for silicon nanowires). VSS and VLS techniques are knownin the art; see, e.g., U.S. patent application publication no.2011/0039690 by Niu et al. entitled “Porous substrates, articles,systems and compositions comprising nanofibers and methods of their useand production,” U.S. Pat. No. 7,825,036 to Yao et al. entitled “Methodof synthesizing silicon wires,” Renard et al. (2010) “Catalystpreparation for CMOS-compatible silicon nanowire synthesis” NatureNanotech 4:654-657, U.S. Pat. No. 7,776,760 to Taylor entitled “Systemsand methods for nanowire growth,” U.S. Pat. No. 7,301,199 to Lieber etal. entitled “Nanoscale wires and related devices,” U.S. Pat. No.7,211,464 to Lieber et al. entitled “Doped elongated semiconductors,growing such semiconductors, devices including such semiconductors andfabricating such devices,” Cui et al. (2001) “Diameter-controlledsynthesis of single-crystal silicon nanowires” Appl. Phys. Lett. 78,2214-2216, Morales et al. (1998) “A laser ablation method for thesynthesis of crystalline semiconductor nanowires” Science 279, 208-211,and Qian et al. (2010) “Synthesis of germanium/multi-walled carbonnanotube core-sheath structures via chemical vapor deposition” in N.Lupu (Ed.), Nanowires Science and Technology (pp. 113-130) Croatia,INTECH. See also Example 1 hereinbelow. For synthesis of siliconnanostructures (e.g., silicon nanowires), the crystallinity of theresulting nanostructures can be controlled, e.g., by controlling thegrowth temperature, precursors, and/or other reaction conditions thatare employed. A chlorinated silane precursor or an etchant gas such asHCl can be employed to prevent undesired deposition of silicon atlocations other than the catalyst (e.g., exposed substrate surfaces orthe sidewall of the reaction chamber) and/or tapering of the nanowiresdue to dripping of molten catalyst down the growing nanowire leading togrowth on the sidewall of the nanowire (which can also result information of an amorphous or polycrystalline shell on the nanowire);see, e.g., U.S. Pat. Nos. 7,776,760 and 7,951,422. These problems aregreatly reduced by use of a solid copper-based catalyst instead of aliquid gold catalyst, so use of a copper-based catalyst can reduce oreliminate need for inclusion of an etchant (or use of a chlorinatedsilane precursor) in the nanostructure synthesis process.

Additional information on nanostructure synthesis using varioustechniques is readily available in the art. See, e.g., U.S. Pat. No.7,105,428 to Pan et al. entitled “Systems and methods for nanowiregrowth and harvesting,” U.S. Pat. No. 7,067,867 to Duan et al. entitled“Large-area nonenabled macroelectronic substrates and uses therefor,”U.S. Pat. No. 7,951,422 to Pan et al. entitled “Methods for orientedgrowth of nanowires on patterned substrates,” U.S. Pat. No. 7,569,941 toMajumdar et al. entitled “Methods of fabricating nanostructures andnanowires and devices fabricated therefrom,” U.S. Pat. No. 6,962,823 toEmpedocles et al. entitled “Methods of making, positioning and orientingnanostructures, nanostructure arrays and nanostructure devices,” U.S.patent application Ser. No. 12/824,485 by Dubrow et al. entitled“Apparatus and methods for high density nanowire growth,” Gudiksen et al(2000) “Diameter-selective synthesis of semiconductor nanowires” J. Am.Chem. Soc. 122, 8801-8802; Gudiksen et al. (2001) “Synthetic control ofthe diameter and length of single crystal semiconductor nanowires” J.Phys. Chem. B 105,4062-4064; Duan et al. (2000) “General synthesis ofcompound semiconductor nanowires” Adv. Mater. 12, 298-302; Cui et al.(2000) “Doping and electrical transport in silicon nanowires” J. Phys.Chem. B 104, 5213-5216; Peng et al. (2000) “Shape control of CdSenanocrystals” Nature 404, 59-61; Puntes et al. (2001) “Colloidalnanocrystal shape and size control: The case of cobalt” Science 291,2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos et al. (Oct. 23, 2001)entitled “Process for forming shaped group III-V semiconductornanocrystals, and product formed using process”; U.S. Pat. No. 6,225,198to Alivisatos et al. (May 1, 2001) entitled “Process for forming shapedgroup II-VI semiconductor nanocrystals, and product formed usingprocess”; U.S. Pat. No. 6,036,774 to Lieber et al. (Mar. 14, 2000)entitled “Method of producing metal oxide nanorods”; U.S. Pat. No.5,897,945 to Lieber et al. (Apr. 27, 1999) entitled “Metal oxidenanorods”; U.S. Pat. No. 5,997,832 to Lieber et al. (Dec. 7, 1999)“Preparation of carbide nanorods”; Urbau et al. (2002) “Synthesis ofsingle-crystalline perovskite nanowires composed of barium titanate andstrontium titanate” J. Am. Chem. Soc., 124, 1186; and Yun et al. (2002)“Ferroelectric Properties of Individual Barium Titanate NanowiresInvestigated by Scanned Probe Microscopy” Nanoletters 2, 447.

Synthesis of core-shell nanostructure heterostructures, namelynanocrystal and nanowire core-shell heterostructures, are described in,e.g., Peng et al. (1997) “Epitaxial growth of highly luminescentCdSe/CdS core/shell nanocrystals with photostability and electronicaccessibility” J. Am. Chem. Soc. 119, 7019-7029; Dabbousi et al. (1997)“(CdSe)ZnS core-shell quantum dots: Synthesis and characterization of asize series of highly luminescent nanocrystallites” J. Phys. Chem. B101, 9463-9475; Manna et al. (2002) “Epitaxial growth and photochemicalannealing of graded CdS/ZnS shells on colloidal CdSe nanorods” J. Am.Chem. Soc. 124, 7136-7145; and Cao et al. (2000) “Growth and propertiesof semiconductor core/shell nanocrystals with InAs cores” J. Am. Chem.Soc. 122, 9692-9702. Similar approaches can be applied to growth ofother core-shell nanostructures. Growth of nanowire heterostructures inwhich the different materials are distributed at different locationsalong the long axis of the nanowire is described in, e.g., Gudiksen etal. (2002) “Growth of nanowire superlattice structures for nanoscalephotonics and electronics” Nature 415, 617-620; Bjork et al. (2002)“One-dimensional steeplechase for electrons realized” Nano Letters 2,86-90; Wu et al. (2002) “Block-by-block growth of single-crystallineSi/SiGe superlattice nanowires” Nano Letters 2, 83-86; and US patentapplication publication no. 2004/0026684 to Empedocles entitled“Nanowire heterostructures for encoding information.” Similar approachescan be applied to growth of other heterostructures.

In embodiments in which the substrate comprises a population ofparticles (e.g., graphite or silica particles), the substrate particleswith catalyst particles disposed thereon are typically loaded into areaction vessel in which nanostructure synthesis is subsequentlyperformed. For example, the substrate particles can be loaded into aquartz tube or cup with a porous frit (e.g., a quartz frit) to retainthe particles, e.g., as gas flows through the vessel during a CVD (e.g.,VLS or VSS) nanostructure synthesis reaction.

The substrate particles can form a packed bed in the reaction vessel.Without limitation to any particular mechanism, conversion of reactant(e.g., a source or precursor gas) depends on the relative reaction andgas flow rates. Minimal variation in reactant concentration throughoutthe bed can be achieved, or high total conversion can be achieved. Inthe case of high total conversion, the amount of nanostructures grown onthe substrate particles typically varies from the entrance to the exitof the packed bed due to depletion of the source gas. This effect can bemitigated, if desired, e.g., by flowing the reactant gas in bothdirections through the vessel or by mixing the substrate particlesduring the growth process.

Where mixing of the substrate particles is desired, the reaction vesselcan contain a mechanical stirrer or mixer that acts to redistribute thesubstrate particles in the vessel over the course of the nanostructuresynthesis reaction. Convection of particles within the bed can alloweach particle to experience similar growth conditions (e.g., temperatureand reactant concentration) on average, particularly when recirculationof particles within the bed is faster than the growth rate of thenanostructures. For example, the reaction vessel can include a helicalribbon or a rotating impeller blade, e.g., in a vertical or horizontalreaction vessel. As another example, the reaction vessel can behorizontal and made to rotate; rotation of the vessel drags thesubstrate particles up the vessel walls, resulting in mixing. Acomponent of the vessel is optionally fixed (i.e., not rotating), forexample, a tube in the center of the vessel for injection of gases.Other components are optionally fixed to the static component, forexample, a scraper to prevent sticking of material to the vessel walls(e.g., a thin band or wire comformal to the vessel walls) or an array ofrigid pins. The reaction vessel can include two linear arrays ofregularly spaced rigid pins, one fixed to the rotating wall and theother to the static inlet tube, with the pins normal to the tube walls.The moving and fixed arrays of pins are offset in an interdigitatedfashion, so that they do not collide but instead push any aggregates ofsubstrate particles between the pins, breaking up and limiting aggregatesize. As additional examples, the substrate particles can be fluidizedby ultrasonic or mechanical shaking of the bed instead of or in additionto by mechanical stirring.

It is worth noting that bed volume typically increases with increasinggas flow rate. It is also worth noting that at very low pressure, forexample, less than about 500 mtorr, fluidization of substrate particlesis impeded. Growth pressure ranges above about 500 mtorr (e.g.,medium-low vacuum, above 200 to 400 torr, to near-atmospheric,atmospheric, or above-atmospheric pressure) are therefore generallypreferred for nanostructure growth in a mixed bed.

The methods can be employed for production of nanostructures for use inany of a variety of different applications. For example, as noted above,the nanostructures and optionally the substrate on which thenanostructures were grown can be incorporated into a battery, batteryanode, and/or battery slurry. In one class of embodiments, thenanostructures and substrate are incorporated into the anode electrodeof a lithium ion battery.

A lithium ion battery typically includes an anode, an electrolyte (e.g.,an electrolyte solution), and a cathode. A separator (e.g., a polymermembrane) is typically placed between the anode and the cathode inembodiments in which the electrolyte is, e.g., a liquid or gel. Inembodiments where a solid-state electrolyte is employed, a separator istypically not included. The anode, electrolyte, cathode, and separator(if present) are encased in a housing.

Suitable materials for the housing, cathode, electrolyte, and separatorare known in the art. See, e.g., U.S. patent application publication no.2010/0297502. For example, the housing can be a metal, polymer, ceramic,composite, or like material, or can include a combination of suchmaterials (e.g., a laminate of metallic and polymer layers). The cathodecan comprise any suitable material known for use as a battery cathode,including, but not limited to, lithium-containing materials such asLiCoO₂, LiFePO₄, LiMnO₂, LiMnO₄, LiNiCoAlO/LiNiCoMnO⁺LiMn₂O₄, LiCoFePO₄and LiNiO₂. The electrolyte can comprise a solid-state electrolyte(e.g., an alkali metal salt, e.g., a lithium salt, mixed with anionically conducting material) or an electrolyte solution (e.g., analkali metal salt, e.g., a lithium salt, e.g., LiPF₆, dissolved in asolvent, e.g. an organic solvent, e.g., diethyl carbonate, ethylenecarbonate, ethyl methyl carbonate, or a combination thereof). Theseparator can be a microporous polymer material having good ionicconductivity and sufficiently low electronic conductivity, e.g., PVDF,polypyrrole, polythiaphene, polyethylene oxide, polyacrylonitrile,poly(ethylene succinate), polypropylene, poly (β-propiolactone), or asulfonated fluoropolymer such as NAFION®.

In one class of embodiments, after nanostructure synthesis, thenanostructures and the substrate are incorporated into a battery slurry,e.g., by mixing the substrate bearing the nanostructures with a polymerbinder and a solvent (e.g., water or an organic solvent). Suitablebinders (e.g., conductive polymers) and solvents are known in the art.Examples include, but are not limited to, polyvinylidene difluoride(PVDF) as the binder with N-methyl-2-pyrrolidone (NMP) as the solvent orcarboxymethyl cellulose (CMC) as the binder with water as the solvent.The battery slurry (which can also be referred to as an active materialslurry) is coated on a current collector, e.g., copper foil. Evaporationof the solvent leaves the active materials (the nanostructures and thesubstrate) and the polymer binder coating the current collector. Thisassembly can then be employed as a battery anode, e.g., after insertioninto a suitable housing along with a cathode, electrolyte, andoptionally a separator placed in the electrolyte between the anode andcathode.

In one class of embodiments, the substrate comprises a population ofgraphite particles and the nanostructures comprise silicon nanowires.Optionally, for example, in embodiments in which the nanowires andgraphite particles are to be incorporated into a battery, slurry, oranode, when nanostructure growth is finished, silicon comprises between1% and 50% of the total weight of the nanostructures and the graphiteparticles, e.g., between 1% and 40%, between 1% and 30%, or between 2%and 25%. Optionally, silicon comprises between 2% and 20% of the totalweight of the nanostructures and the graphite particles, or between 6%and 20% of the total weight of the nanostructures and the graphiteparticles. Where the nanowires and graphite are used in a battery, itwill be evident that increasing the silicon content tends to increasethe capacity per gram and that the silicon content is desirably matchedwith the specific capacity of the cathode.

Battery Active Materials Synthesized Using Copper-Based Catalysts

Compositions produced by or useful in practicing any of the methodsherein are also a feature of the invention. For example, nanostructuresgrown from copper-based catalysts on porous substrates are a feature ofthe invention.

Accordingly, one general class of embodiments provides a compositionthat includes a porous substrate and a population of nanostructuresattached thereto. A region of each of the nanostructures is attached tothe substrate, and another region of each nanostructure (generallydistal to the first region) comprises copper, a copper compound, and/ora copper alloy, equivalent to or derived from the catalyst that wasemployed in the synthesis reaction. Attachment of the nanostructures tothe substrate on which they were grown is typically through van derWaals interactions.

Thus, in one class of embodiments, the composition includes a poroussubstrate and a population of silicon nanowires attached thereto,wherein one end of a member nanowire is attached to the substrate andthe other end of the member nanowire comprises copper, a coppercompound, and/or a copper alloy.

Essentially all of the features noted for the embodiments above apply tothese embodiments as well, as relevant; for example, with respect totype, composition, and size of nanostructures, composition andconfiguration of the substrate, catalyst material, incorporation into abattery slurry, battery anode, or battery, and/or the like.

For example, the composition can include nanowires having an averagediameter of about 10 nm to about 500 nm, or about 20 nm to about 400 nm,about 20 nm to about 300 nm, about 20 nm to about 200 nm, about 20 nm toabout 100 nm, about 30 nm to about 100 nm, or about 40 nm to about 100nm. Typically, the nanowires have an average diameter less than about150 nm, e.g., between about 10 nm and about 100 nm, e.g., between about30 nm and about 50 nm, e.g., between about 40 nm and about 45 nm. Thenanowires are optionally less than about 100 μm in length, e.g., lessthan about 10 μm, about 100 nm to about 100 μm, or about 1 μm to about75 μm, about 1 μm to about 50 μm, or about 1 μm to about 20 μm inlength. The aspect ratios of the nanowires are optionally up to about2000:1 or about 1000:1. For example, the nanowires can have a diameterof about 20 nm to about 200 nm and a length of about 0.1 μm to about 50μm. The nanowires can comprise monocrystalline silicon, polycrystallinesilicon, amorphous silicon, or a combination thereof, e.g., in therelative amounts detailed above. For example, the nanowires optionallycomprise a monocrystalline core and a shell layer, wherein the shelllayer comprises amorphous silicon, polycrystalline silicon, or acombination thereof.

As for the embodiments above, the porous substrate is optionally a mesh,fabric, e.g., a woven fabric (e.g., a carbon fabric), or fibrous mat. Inpreferred embodiments, the substrate comprises a population ofparticles, sheets, fibers (including, e.g., nanofibers), and/or thelike. Thus, exemplary substrates include a plurality of silica particles(e.g., a silica powder), a plurality of carbon sheets, carbon powder ora plurality of carbon particles, natural and/or artificial (synthetic)graphite, natural and/or artificial (synthetic) graphite particles,graphene, graphene powder or a plurality of graphene particles, carbonfibers, carbon nanostructures, carbon nanotubes, and carbon black. Wherethe nanostructures, e.g., silicon nanowires, are intended for use as abattery active material, the substrate is typically a carbon-basedsubstrate, for example, a population of graphite particles.

In embodiments in which the substrate comprises a population ofparticles (e.g., graphite particles), the particles can be ofessentially any desired shape, for example, spherical or substantiallyspherical, elongated, oval/oblong, plate-like (e.g., plates, flakes, orsheets), and/or the like. Similarly, the substrate particles can be ofessentially any size. Optionally, the substrate particles have anaverage diameter between about 0.5 μm and about 50 μm, e.g., betweenabout 0.5 μm and about 2 μm, between about 2 μm and about 10 μm, betweenabout 2 μm and about 5 μm, between about 5 μm and about 50 μm, betweenabout 10 μm and about 30 μm, between about 10 μm and about 20 μm,between about 15 μm and about 25 μm, between about 15 μm and about 20μm, or about 20 μm. In one exemplary class of embodiments, thenanostructures are silicon nanowires, the substrate is a population ofgraphite particles, and the graphite particle size is about 10-20 μm. Inanother exemplary class of embodiments, the nanostructures are siliconnanowires, the substrate is a population of graphite particles, thegraphite particle size is a few μm (e.g., about 2 μm or less), and thegraphite particles are optionally spherical.

The catalyst-derived material on the distal region of thenanostructures, e.g., the ends of the member nanowires not attached tothe substrate, can comprise, e.g., elemental copper, copper oxide,copper silicide, or a combination thereof.

The composition optionally includes a polymer binder (for example, anyof those noted above) and/or a solvent (e.g., water or an organicsolvent as noted above). In one class of embodiments, for example, wherethe composition is included in a battery, slurry, or anode, thesubstrate comprises a population of graphite particles, thenanostructures are silicon nanostructures, and silicon comprises between1% and 50% of the total weight of the nanostructures and the graphiteparticles, e.g., between 1% and 40%, between 1% and 30%, or between 2%and 25%. Optionally, silicon comprises between 2% and 20% of the totalweight of the nanostructures and the graphite particles, or between 6%and 20% of the total weight of the nanostructures and the graphiteparticles.

A battery slurry comprising a composition of the invention is also afeature of the invention. Thus, one class of embodiments provides abattery slurry comprising a porous substrate, a population of siliconnanowires, a polymer binder, and a solvent, where one end of a membernanowire is attached to the substrate and the other end of the membernanowire comprises copper, a copper compound, and/or a copper alloy.Similarly, a battery anode comprising a composition of the invention isalso a feature of the invention, as is a battery comprising acomposition of the invention. Thus, one class of embodiments provides abattery (e.g., a lithium ion battery) that includes an anode comprisinga porous substrate and a population of silicon nanowires attachedthereto, wherein one end of a member nanowire is attached to thesubstrate and the other end of the member nanowire comprises copper, acopper compound, and/or a copper alloy. A polymer binder is typicallyalso included in the anode, and the battery typically also includes acathode, an electrolyte, and an optional separator, encased in ahousing. Suitable binders, cathode materials, electrolytes, housingmaterials, and the like have been noted hereinabove.

Colloidal Synthesis of Copper-Based Nanoparticles

Copper-containing nanoparticles are of interest in a wide variety ofapplications. For example, cuprous oxide (Cu₂O) is a p-typesemiconductor with potential applications in solar energy conversion,catalysis (e.g., CO oxidation and photoactivated water splitting into H₂and O₂), and gas sensing and as an anode material for lithium ionbatteries. As noted above, cuprous oxide and other copper-basednanoparticles are also of interest as catalysts for nanostructuresynthesis.

Nanoparticles containing copper, copper alloy, and/or a copper compoundcan be conveniently prepared using colloidal synthesis techniques. Asused herein, synthesis of colloidal nanoparticles refers to productionof a colloid mixture that includes a solvent and nanoparticles, wherethe particles are dispersed evenly throughout the solvent. “Colloidalnanoparticles” refers to nanoparticles produced via colloidal synthesis,even if the nanoparticles are subsequently removed from the solvent, forexample, by deposition on a solid substrate.

Cu₂O nanoparticles can be synthesized from a modified Fehling'ssolution, where Cu²⁺, protected by a capping agent, is reduced to Cu₂Oin an alkaline aqueous solution. Nanoparticles that precipitate out ofthe solution when synthesized via this route can be resuspended to forma colloid if appropriate ligands are attached to the nanoparticles. Forexample, Cu₂O nanoparticles can be synthesized as a stable Cu₂O colloidin water (i.e., a hydrosol), using sodium ascorbate or ascorbic acid toreduce a copper (II) salt to Cu₂O in the presence of surfactant.Controlling conditions such as pH and concentrations results information of colloidal Cu₂O (stable for at least several hours), with anarrow particle size distribution and average particle size varying,e.g., from 15 to 100 nm as determined by electron microscopy and/orlight scattering. In particular, colloidal Cu₂O, which is stable for afew hours, has been synthesized from the reduction of copper (II)sulfate using sodium ascorbate or ascorbic acid in the presence ofsodium dodecyl sulfate (an anionic surfactant), Triton X-100 (a nonionicsurfactant), or cetrimonium chloride (a cationic surfactant). FIG. 1Panels A and B show colloidal Cu₂O nanoparticles synthesized in thismanner; the average size was 33 nm as measured for 50 particles byelectron microscopy, while results from dynamic light scatteringmeasurements show a strong peak around 50 nm. (In general, particle sizeas measured by light scattering tends to be greater than that measuredby electron microscopy, since light scattering measures the hydrodynamicdiameter which depends on ionic strength of the suspension and surfacestructure of the adsorbed surfactant layer.) If the solution pH is toohigh, the Cu₂O particles tend to grow excessively, resulting inprecipitates. On the other hand, if the pH is too low, small Cu₂Oparticles tend to dissolve in water and the colloid becomes unstable.When both pH and concentrations are appropriate and the Cu₂O surfacesare protected by suitable functional groups, a stable Cu₂O hydrosolexists, with a unique color ranging from light green to golden yellow.For example, colloidal Cu₂O, which is stable for at least one hour, maybe obtained in a pH range of 8-11, with up to 5 millimolar copper ions.

As will be evident, various combinations of copper salt, reducing agent,capping agent, and surfactant can be employed. Exemplary reducing agentsinclude, but are not limited to, glucose, formaldehyde, ascorbic acid,and phosphorous acid. Exemplary capping agents include, but are notlimited to, tartaric acid and sodium citrate. Suitablesurfactants/dispersants include, but are not limited to, nonionicsurfactants (e.g., Triton X-100), cationic surfactants (e.g.,cetrimonium chloride or cetrimonium bromide (CTAB)), anionic surfactants(e.g., sodium dodecyl sulfate (SDS)), and polymers such as polyethyleneglycol (e.g., molecular weight 600-8000) and polyvinylpyrrolidone (e.g.,molecular weight 55,000).

Copper-containing nanoparticles, including Cu₂O nanoparticles, can alsobe synthesized in non-aqueous solutions. See, e.g., Yin et al. (2005)“Copper oxide nanocrystals” J Am Chem Soc 127:9506-9511 and Hung et al.(2010) “Room-temperature formation of hollow Cu₂O nanoparticles” AdvMater 22:1910-1914.

Additional information on colloidal synthesis of nanoparticles isavailable in the art. See, e.g., Kuo et al. (2007) “Seed-mediatedsynthesis of monodispersed Cu₂O nanocubes with five different sizeranges from 40 to 420 nm” Adv. Funct. Mater. 17:3773-3780 and Kooti andMatouri (2010) “Fabrication of nanosized cuprous oxide using Fehling'ssolution” Transaction F: Nanotechnology 17(1):73-78 for synthesis ofCu₂O and Yu et al. (2009) “Synthesis and characterization ofmonodispersed copper colloids in polar solvents” Nanoscale Res Lett.4:465-470 for synthesis of elemental copper colloids.

The colloidal nanoparticles are optionally employed as catalystparticles for nanostructure synthesis. See, for example, FIG. 2 Panels Aand B, which show colloidal Cu₂O nanoparticles deposited on a graphiteparticle substrate and silicon nanowires grown from the Cu₂Onanoparticles. The resulting nanowires are similar to nanowiresconventionally grown using gold catalyst particles.

Thus, one general class of embodiments provides methods for synthesizingnanostructures. In the methods, colloidal nanoparticles comprisingcopper and/or a copper compound are synthesized and deposited on asubstrate, and nanostructures are grown from the nanoparticles.Exemplary nanostructures and materials are noted hereinabove andinclude, but are not limited to, silicon nanowires.

Accordingly, in one class of embodiments, colloidal nanoparticlescomprising copper and/or a copper compound are synthesized. After theirsynthesis, the nanoparticles are deposited on a substrate. Nanowires arethen grown from the nanoparticles.

Suitable substrates include a planar substrate, silicon wafer, or foil(e.g., a metal foil, e.g., stainless steel foil). Suitable substratesinclude nonporous substrates as well as porous substrates such as thosedescribed above, e.g., a mesh, fabric, e.g., a woven fabric (e.g., acarbon fabric), fibrous mat, population of particles, sheets, fibers(including, e.g., nanofibers), and/or the like. Thus, exemplarysubstrates include a plurality of silica particles (e.g., a silicapowder), a plurality of carbon sheets, carbon powder or a plurality ofcarbon particles, natural and/or artificial (synthetic) graphite,natural and/or artificial (synthetic) graphite particles, graphene,graphene powder or a plurality of graphene particles, carbon fibers,carbon nanostructures, carbon nanotubes, and carbon black. Where thenanostructures, e.g., silicon nanowires, are intended for use as abattery active material, the substrate is typically a carbon-basedsubstrate, for example, a population of graphite particles.

In embodiments in which the substrate comprises a population ofparticles (e.g., graphite particles), the particles can be ofessentially any desired shape, for example, spherical or substantiallyspherical, elongated, oval/oblong, plate-like (e.g., plates, flakes, orsheets), and/or the like. Similarly, the substrate particles can be ofessentially any size. Optionally, the substrate particles have anaverage diameter between about 0.5 μm and about 50 μm, e.g., betweenabout 0.5 μm and about 2 μm, between about 2 μm and about 10 μm, betweenabout 2 μm and about 5 μm, between about 5 μm and about 50 μm, betweenabout 10 μm and about 30 μm, between about 10 μm and about 20 μm,between about 15 μm and about 25 μm, between about 15 μm and about 20μm, or about 20 μm. In one exemplary class of embodiments, thenanostructures are silicon nanowires, the substrate is a population ofgraphite particles, and the graphite particle size is about 10-20 μm. Inanother exemplary class of embodiments, the nanostructures are siliconnanowires, the substrate is a population of graphite particles, thegraphite particle size is a few μm (e.g., about 2 μm or less), and thegraphite particles are optionally spherical.

The copper compound is optionally copper oxide. In one class ofembodiments, the nanoparticles comprise elemental copper (Cu), copper(I) oxide (Cu₂O), copper (II) oxide (CuO), or a combination thereofOptionally, the substrate comprises a population of graphite particles.

The shape and size of the nanoparticles can vary, for example, dependingon the diameter desired for the resulting nanowires. For example, thenanoparticles optionally have an average diameter between about 5 nm andabout 100 nm, e.g., between about 10 nm and about 100 nm, between about20 nm and about 50 nm, or between about 20 nm and about 40 nm.Optionally, the nanoparticles have an average diameter of about 20 nm.The nanoparticles can be of essentially any shape, including, but notlimited to, spherical, substantially spherical, or other regular orirregular shapes.

Essentially all of the features noted for the embodiments above apply tothese embodiments as well, as relevant; for example, with respect tonanostructure growth technique (e.g., VLS or VSS), type, composition,and size of the resulting nanostructures, ratio of nanostructures tosubstrate (e.g., silicon to graphite) by weight, incorporation into abattery slurry, battery anode, or battery, and/or the like.

The nanoparticles can be deposited on the substrate using essentiallyany convenient technique, for example, spin coating, spraying, dipping,or soaking. Nanoparticles can be deposited on a particulate substrate(e.g., graphite particles) by stirring a mixture of the nanoparticlesand substrate particles, e.g., in a solvent; the substrate particleswith the nanoparticles deposited thereon can then be recovered byfiltration and optionally dried to remove residual solvent. Preferably,the nanoparticles are deposited on the substrate surface and their phaseand/or morphology is preserved before nanostructure synthesis iscommenced (e.g., by introduction of silane gas into the reactionchamber). Deposition of the nanoparticles on the substrate is desirablyeven.

The colloid and/or the substrate can be treated or modified to enhanceassociation of the nanoparticles with the surface of the substrate.Where the colloid is negatively charged and the substrate is positivelycharged (or vice versa), the nanoparticles generally stick to thesurface of the substrate well. If necessary, however, the charge orother surface characteristics of either or both the colloid and thesurface can be modified. For example, where elemental copper or copperoxide nanoparticles are deposited on graphite particles as thesubstrate, the graphite can be treated, e.g., with acid, to increaseadherence of the nanoparticles to the graphite. Ligands on thenanoparticles (e.g., surfactants) and/or the substrate can be varied.

Electroless Deposition of Copper-Based Nanoparticles

Electroless plating generally involve immersion of a substrate in asolution containing a metal salt. Chemical reactions on the substrate'ssurface result in plating of the metal out of solution onto the surface.The reactions occur without use of external electrical power.

For example, electroless copper plating can be achieved by immersing anactivated substrate in a bath containing a copper source (e.g., a copper(II) salt), a chelating agent, and a reducing agent such asformaldehyde, typically at alkaline pH. Without limitation to anyparticular mechanism, deposition of copper can occur through thefollowing reactions:

Cu[L]_(x) ²⁺+2e ⁻→Cu+xL  Cathodic:

2HCHO+4OH⁻→2HCOO⁻+H₂+2H₂O+2e ⁻  Anodic:

2HCHO+4OH⁻+Cu[L]_(x) ²⁺→Cu+2HCOO⁻+H₂+2H₂O+xL  Overall:

The reaction is autocatalytic only in the presence of an activatedsurface or when hydrogen is being generated. Deposition rate depends on,e.g., the copper complexing agent, reducing agent, pH, bath temperature,and any additives that may be included in the plating solution (e.g.,stabilizer, surfactant, accelerator, etc.).

Electroless plating has been used to plate or deposit a thin layer ofcopper film on substrates, particularly dielectric materials such as PCB(plastic circuit board). The process is, however, not user-friendly,particularly when the substrate is in the form of fine powders whichrequire filtration to separate the solid from the liquid effluent. Toachieve the desired copper level, plating parameters (e.g., time ofimmersion and bath temperature) must be precisely controlled. Inaddition, critical ingredients in the plating bath must be analyzed andreplenished to maintain the process stability. See, e.g., U.S. Pat. No.4,136,216 to Feldstein entitled “Non-precious metal colloidaldispersions for electroless metal deposition”; U.S. Pat. No. 4,400,436to Breininger et al. entitled “Direct electroless deposition of cuprousoxide films”; U.S. Pat. No. 4,171,225 to Molenaar et al. entitled“Electroless copper plating solutions”; Liu et al. (1999) “Modificationsof synthetic graphite for secondary lithium-ion battery applications” JPower Sources 81-82:187-191; Lu et al. (2002) “Electrochemical andthermal behavior of copper coated type MAG-20 natural graphite”Electrochimica Acta 47(10):1601-1606; Yu (2007) “A novel processingtechnology of electroless copper plating on graphite powder” MaterialsProtection 2007-09; Bindra and White (1990) “Fundamental aspects ofelectroless copper plating” in Mallory and Hajdu (Eds.) ElectrolessPlating—Fundamentals and Applications (pp. 289-329) William AndrewPublishing/Noyes; Li and Kohl (2004) “Complex chemistry & theelectroless copper plating process” Plating & Surface Finishing February2-8; Xu et al. (2004) “Preparation of copper nanoparticles on carbonnanotubes by electroless plating method” Materials Research Bulletin39:1499-1505; and Siperko (1991) “Scanning tunneling microscopy studiesof Pd—Sn catalyzed electroless Cu deposited on pyrolytic graphite” J VacSci Technol A 9(3):1457-1460. Achieving consistent and reproducibledeposition of discrete nanoparticles (rather than a continuous orsemi-continuous film) on the surface of the substrate is even morechallenging.

In one aspect, the present invention overcomes the above noteddifficulties by providing methods of producing copper-basednanoparticles on any of a variety of substrates. The amount of thecopper source present in the plating solution is much lower than istypical in conventional plating processes. In addition, in contrast toconventional techniques, the copper source is completely depleted fromthe bath after deposition. Deposition of copper at a high copperconcentration requires careful monitoring of time and reactantconcentrations and replenishment of the bath components; deposition atlow concentration, as described herein, provides more convenient controlover the reaction.

The inventive methods have a number of advantages. For example, since aknown amount of the copper source is added to the plating bath andsubstantially completely depleted from the bath by deposition on thesubstrate, the reaction stops automatically, and the amount of copperdeposited on the substrate is known and precisely controlled. There isno need to critically control plating parameters such as time and bathtemperature to achieve the desired percentage of copper deposited. Thereis no need to analyze and/or replenish depleted chemicals during theplating process, which greatly simplifies deposition on particulatesubstrates, whose high surface area complicates deposition withconventional techniques. Where powdered substrates are employed, thesubstrate and the plating chemicals can be mixed in various orders tocontrol the deposition rate and uniformity. Since copper issubstantially completely depleted after deposition, treatment of spentplating solutions prior to waste disposal is simplified.

Accordingly, one general class of embodiments provides methods forproducing nanoparticles. In the methods, a substrate is provided. Alsoprovided is an electroless plating solution that comprises at most 10millimolar copper ions (e.g., Cu²⁺ and/or Cu⁺). The substrate isimmersed in the plating solution, which is optionally heated to about60-70° C., whereby the copper ions from the plating solution formdiscrete nanoparticles comprising copper and/or a copper compound on thesubstrate, until the plating solution is substantially completelydepleted of copper ions. “Substantially completely depleted of copperions” means that less than 5 ppm or even less than 1 ppm copper ionremains in the solution. No monitoring or analysis of the components ofthe plating solution is necessary during the immersion step, and noreagents need to be added to the plating solution during the immersionstep.

Suitable substrates include a planar substrate, silicon wafer, or foil(e.g., a metal foil, e.g., stainless steel foil). Suitable substratesinclude nonporous substrates as well as porous substrates such as thosedescribed above, e.g., a mesh, fabric, e.g., a woven fabric (e.g., acarbon fabric), fibrous mat, population of particles, sheets, fibers(including, e.g., nanofibers), and/or the like. Thus, exemplarysubstrates include a plurality of silica particles (e.g., a silicapowder), a plurality of carbon sheets, carbon powder or a plurality ofcarbon particles, natural and/or artificial (synthetic) graphite,natural and/or artificial (synthetic) graphite particles, graphene,graphene powder or a plurality of graphene particles, carbon fibers,carbon nanostructures, carbon nanotubes, and carbon black. The substrateis optionally a carbon-based substrate, for example, a population ofgraphite particles.

In embodiments in which the substrate comprises a population ofparticles (e.g., graphite particles), the particles can be ofessentially any desired shape, for example, spherical or substantiallyspherical, elongated, oval/oblong, plate-like (e.g., plates, flakes, orsheets), and/or the like. Similarly, the substrate particles can be ofessentially any size. Optionally, the substrate particles have anaverage diameter between about 0.5 μm and about 50 μm, e.g., betweenabout 0.5 μm and about 2 μm, between about 2 μm and about 10 μm, betweenabout 2 μm and about 5 μm, between about 5 μm and about 50 μm, betweenabout 10 μm and about 30 μm, between about 10 μm and about 20 μm,between about 15 μm and about 25 μm, between about 15 μm and about 20μm, or about 20 μm. In one exemplary class of embodiments, the substrateis a population of graphite particles, and the graphite particle size isabout 10-20 μm. In another exemplary class of embodiments, the substrateis a population of graphite particles, the graphite particle size is afew μm (e.g., about 2 μm or less), and the graphite particles areoptionally spherical.

The substrate is typically activated prior to its immersion in theplating solution. The substrate is optionally activated as known in theart by soaking it in a solution of a metal salt, e.g., by soaking in asolution of PdCl₂ or AgNO₃. Graphite substrates, however, particularlygraphite particles which have a very high surface area compared to aconventional planar substrate, are conveniently activated simply byheating them prior to immersion in the plating solution. Thus, themethods optionally include activating the substrate (e.g., a populationof graphite particles) by heating it to 20° C. or more prior toimmersing it in the plating solution. Optionally, the substrate isactivated by heating it to 30° C. or more, preferably 40° C. or more,50° C. or more, or 60° C. or more prior to immersion.

In embodiments in which the substrate comprises a population ofparticles, the methods can include filtering the plating solution torecover the substrate particles from the plating solution after theplating solution is substantially completely depleted of copper ions. Ingeneral, the substrate is typically removed from the plating solutionafter deposition is complete (that is, after the plating solution issubstantially completely depleted of copper ions).

As described above, electroless plating generally involves chemicalreduction from an aqueous plating solution that includes one or morecopper source, complexing or chelating agent, and reducing agent,optionally at alkaline pH. The copper source is typically a copper salt,e.g., a copper (II) salt (for example, copper sulfate, copper nitrate,copper (II) chloride (CuCl₂), or copper acetate) or a copper (I) salt(for example, copper (I) chloride (CuCl)). Exemplary chelating agentsinclude, but are not limited to, Rochelle salt, EDTA, and polyols (e.g.,Quadrol® (N,N,N′,N′-tetrakis (2-hydroxypropyl) ethylene-diamine)).Exemplary reducing agents include, but are not limited to, formaldehydeand sodium hypophosphite (NaH₂PO₂). The plating solution optionallyincludes one or more additives such as a stabilizer, surfactant, and/oraccelerator. The pH of the plating solution can be adjusted as is wellknown in the art, for example, by addition of sodium hydroxide (NaOH),typically to a pH of 12 to 13. The plating solution is optionallyheated, e.g., to a temperature of 60-70° C.

In one exemplary class of embodiments, the plating solution comprises acopper (II) salt (e.g., less than 1 g/L anhydrous copper sulfate),Rochelle salt, and formaldehyde and has an alkaline pH. In this class ofembodiments, the substrate optionally comprises a population ofparticles (e.g., graphite particles).

As noted, when the substrate is initially immersed in the platingsolution, the plating solution comprises at most 10 millimolar copperions. Optionally, the plating solution comprises at most 8 millimolarcopper ions, at most 6 millimolar copper ions, at most 4 millimolarcopper ions, or at most 2 millimolar copper ions. It will be evidentthat the plating solution initially comprises substantially more than 5ppm copper ions; for example, the initial concentration of copper ionscan be at least 2 millimolar, e.g., at least 4 millimolar, at least 6millimolar, or at least 8 millimolar.

As noted, the resulting nanoparticles can include copper or a coppercompound (for example, copper oxide). In one class of embodiments, thenanoparticles comprise elemental copper (Cu), copper (I) oxide (Cu₂O),copper (II) oxide (CuO), or a combination thereof. Since copper oxidizesreadily into copper oxide in air, where copper nanoparticles aredeposited the nanoparticles can also contain at least some copper oxideunless protected from oxidation after the deposition.

The resulting nanoparticles optionally have an average diameter betweenabout 5 nm and about 100 nm, e.g., between about 10 nm and about 100 nm,between about 20 nm and about 50 nm, or between about 20 nm and about 40nm. Optionally, the nanoparticles have an average diameter of about 20nm. The nanoparticles can be of essentially any shape, but are typicallyirregularly shaped.

The resulting nanoparticles are optionally employed as catalystparticles for subsequent synthesis of other nanostructures, e.g.,nanowires. Thus, the methods can include, after the plating solution issubstantially completely depleted of copper ions, removing the substratefrom the plating solution and then growing nanostructures (e.g.,nanowires, e.g., silicon nanowires) from the nanoparticles on thesubstrate. See, for example, FIG. 3 Panels A and B, which show coppernanoparticles deposited on a graphite particle substrate from anelectroless plating solution and silicon nanowires grown from the coppernanoparticles.

Essentially all of the features noted for the embodiments above apply tothese embodiments as well, as relevant; for example, with respect tonanostructure growth technique (e.g., VLS or VSS), type, composition,and size of the resulting nanostructures, ratio of nanostructures tosubstrate (e.g., silicon to graphite) by weight, incorporation into abattery slurry, battery anode, or battery, and/or the like.

The plating solution can be employed as a single use bath or as areusable bath. Thus, in one class of embodiments, after the solution isprepared, the substrate is immersed in the solution until the platingsolution is substantially completely depleted of copper ions, and thesolution is then disposed of (after any necessary treatment to render itsafe for disposal) rather than being replenished and reused for a secondsubstrate. In other embodiments, however, the bath is reused.

Thus, in one class of embodiments, after the plating solution issubstantially completely depleted of copper ions, the substrate isremoved from the plating solution, then copper ions are added to theplating solution, and then a second substrate is immersed in the platingsolution. Typically, the same type and amount of copper source are addedto replenish the reagents that were used to prepare the solutioninitially. Thus, as one example, adding copper ions to the platingsolution can comprise adding a copper (II) salt to the plating solution;additional exemplary copper sources are listed hereinabove. Typically,after addition of the copper ions, the plating solution again comprisesat most 10 millimolar copper ions. The second substrate is typically butneed not be of the same type as the first substrate, e.g., a secondpopulation of particles, e.g., graphite particles.

Where the plating solution is to be reused a large number of times, theconcentration of reducing agent and/or chelating agent and the pH can beanalyzed and adjusted if necessary. Where only a short bath life isneeded, however, or where the plating solution is used only once, thereis no need to analyze the solution and replenish depleted reagents(other than the copper source, if the solution is used more than once).The shorter bath life results in ease of operation.

Since the copper source is nearly completely depleted after deposition,disposal of used plating solution is simplified compared to conventionaltechniques in which a large amount of copper remains in the usedsolution. Waste neutralization can be performed in situ. For example, inembodiments in which the plating solution comprises formaldehyde, afterthe plating solution is substantially completely depleted of copper ionsthe formaldehyde can be treated by addition of sodium sulfite to theplating solution prior to disposing of the solution. The treatment withsodium sulfite can be performed before or after removal of the substratefrom the plating solution, e.g., before or after filtration to recover aparticulate substrate. The waste can then be safely disposed of after pHneutralization.

Additional information on treatment of plating solutions prior todisposal is available in the art. See, e.g., Capaccio (1990) “Wastewatertreatment for electroless plating” in Mallory and Hajdu (Eds.)Electroless Plating—Fundamentals and Applications (pp. 519-528) WilliamAndrew Publishing/Noyes.

As noted above, nanoparticles produced by electroless deposition can beemployed as catalyst particles in subsequent nanostructure synthesisreactions. Accordingly, one general class of embodiments providesmethods for producing nanostructures (e.g., nanowires). In the methods,a substrate is provided. An electroless plating solution comprisingcopper ions is also provided, and the substrate is immersed in theplating solution, which is optionally heated to about 60-70° C., wherebythe copper ions from the plating solution form discrete nanoparticlescomprising copper and/or a copper compound on the substrate.Nanostructures (e.g., nanowires, e.g., silicon nanowires) are then grownfrom the nanoparticles on the substrate. Typically the substrate isremoved from the plating solution prior to growth of the nanostructures.

As detailed above, limiting the concentration of copper ions in theplating solution can be advantageous, particularly for deposition onporous and/or particulate substrates having high surface areas. Thus,optionally the plating solution comprises at most 10 millimolar copperions (e.g., Cu²⁺ and/or Cu⁺). Essentially all of the features noted forthe embodiments above apply to these embodiments as well, as relevant;for example, with respect to type and composition of substrate(nonporous, porous, particles, graphite particles, sheets, wafers,etc.), activation of the substrate, size, shape, and composition of thenanoparticles (e.g., elemental copper and/or copper oxide), componentsof the plating solution (copper source and reducing, chelating, andother reagents), filtration step to recover a particulate substrate,reuse versus single use of the plating solution, nanostructure growthtechnique (e.g., VLS or VSS), type, composition, and size of theresulting nanostructures, ratio of nanostructures to substrate (e.g.,silicon to graphite) by weight, incorporation into a battery slurry,battery anode, or battery, and/or the like.

Analogous methods to those detailed for copper apply to electrolessdeposition of nickel nanoparticles. Such methods are also a feature ofthe invention. Accordingly, one general class of embodiments providesmethods for producing nanoparticles. In the methods, a substrate isprovided. Also provided is an electroless plating solution thatcomprises at most 10 millimolar nickel ions. The substrate is immersedin the plating solution, whereby the nickel ions from the platingsolution form discrete nanoparticles comprising nickel and/or a nickelcompound on the substrate, until the plating solution is substantiallycompletely depleted of nickel ions. Essentially all of the featuresnoted for the embodiments above apply to these embodiments as well, asrelevant.

Formation of Copper-Based Nanoparticles Through Adsorption

Nanoparticles can be conveniently formed by adsorption of copper ions ora copper complex onto the surface of a substrate. Adsorption isgenerally defined as the process through which a substance initiallypresent in one phase (e.g., a liquid) is removed from that phase byaccumulation at the interface between that phase and a separate phase(e.g., a solid). Adsorption is a physical separation process in whichthe adsorbed material is not chemically altered. Nanoparticles producedvia adsorption can be employed as catalysts for subsequent nanostructuregrowth.

Accordingly, one general class of embodiments provides methods forproducing nanostructures (e.g., nanowires). In the methods, a substrateis provided. A solution comprising copper ions and/or a copper complexis also provided, and the substrate is immersed in the solution, wherebythe copper ions and/or the copper complex are adsorbed on the surface ofthe substrate, thereby forming discrete nanoparticles comprising acopper compound on the surface of the substrate. Nanostructures (e.g.,nanowires, e.g., silicon nanowires) are then grown from thenanoparticles on the substrate. Typically the substrate is removed fromthe solution prior to growth of the nanostructures.

The solution optionally includes a copper (I) salt or a copper (II)salt, for example, copper sulfate, copper acetate, or copper nitrate.The solution can include a copper complex comprising a chelating agent(a polydentate ligand that forms two or more coordinate bonds to themetal in the complex), for example, copper (II) tartrate or copperethylenediaminetetraacetate (EDTA). The copper complex can be preformedprior to its addition to the solution, or it can be formed in thesolution, for example, by mixing a copper salt and a chelating agent(e.g., Rochelle salt and a copper (II) salt to form copper (II)tartrate). Employing a copper complex, particularly a complex comprisingan organic or other nonpolar ligand that has stronger van der Waalsinteractions with the surface of the substrate than do copper ions,typically results in greater adsorption on carbon-based substrates thanis seen with uncomplexed copper ions.

In one class of embodiments the solution is an aqueous solution,typically, an alkaline solution. Optionally, the solution has a pH of 12to 13. In another class of embodiments, the solution includes an organicsolvent (e.g., hexane) instead of water.

As noted above, the copper ion or complex does not undergo any chemicalreaction when it is adsorbed to form the nanoparticles. Thus thesolution does not include a reducing agent, in contrast to the platingsolution employed in the electroless deposition techniques describedabove.

Suitable substrates include a planar substrate, nonporous substrate,silicon wafer, or foil (e.g., a metal foil, e.g., stainless steel foil),but more preferably include porous substrates having high surface areassuch as those described above, e.g., a mesh, fabric, e.g., a wovenfabric (e.g., a carbon fabric), fibrous mat, population of particles,sheets, fibers (including, e.g., nanofibers), and/or the like. Thus,exemplary substrates include a plurality of silica particles (e.g., asilica powder), a plurality of carbon sheets, carbon powder or aplurality of carbon particles, natural and/or artificial (synthetic)graphite, natural and/or artificial (synthetic) graphite particles,graphene, graphene powder or a plurality of graphene particles, carbonfibers, carbon nanostructures, carbon nanotubes, and carbon black. Thesubstrate is optionally a carbon-based substrate, for example, apopulation of graphite particles.

In embodiments in which the substrate comprises a population ofparticles (e.g., graphite particles), the particles can be ofessentially any desired shape, for example, spherical or substantiallyspherical, elongated, oval/oblong, plate-like (e.g., plates, flakes, orsheets), and/or the like. Similarly, the substrate particles can be ofessentially any size. Optionally, the substrate particles have anaverage diameter between about 0.5 μm and about 50 μm, e.g., betweenabout 0.5 μm and about 2 μm, between about 2 μm and about 10 μm, betweenabout 2 μm and about 5 μm, between about 5 μm and about 50 μm, betweenabout 10 μm and about 30 μm, between about 10 μm and about 20 μm,between about 15 μm and about 25 mm, between about 15 μm and about 20mm, or about 20 mm. In one exemplary class of embodiments, the substrateis a population of graphite particles, and the graphite particle size isabout 10-20 mm. In another exemplary class of embodiments, the substrateis a population of graphite particles, the graphite particle size is afew μm (e.g., about 2 μm or less), and the graphite particles areoptionally spherical.

In embodiments in which the substrate comprises a population ofparticles, the methods can include filtering the solution to recover thesubstrate particles from the solution after deposition is complete(e.g., after the nanoparticles have reached a desired size, the desiredamount of copper has been adsorbed on the substrate, or the solution issubstantially completely depleted of copper ions and/or complex).

The solution is optionally heated to a temperature at which the ligandin the copper complex is stable, e.g., to 60-70° C., to increaseadsorption. After formation of the nanoparticles and removal of thesubstrate from the solution, the substrate can be further heated, e.g.,to a temperature above which the ligand in the complex is stable, todecompose the copper compound constituting the nanoparticles and yieldcopper oxide or (if heating is performed in a reducing atmosphere)elemental copper nanoparticles. Such heating can be a separate step, butmore typically occurs in the course of nanostructure synthesis from thenanoparticles.

The concentration of copper ions and/or complex in the solution can bevaried as desired. However, limiting the amount of copper present in thesolution can be advantageous, since disposal of used solution issimplified if copper is substantially completely depleted from thesolution by formation of the nanoparticles, as noted for the electrolessdeposition techniques above. Thus, in one class of embodiments, thesolution comprises at most 10 millimolar copper ions or atoms.Optionally, the solution comprises at most 8 millimolar copper, at most6 millimolar copper, at most 4 millimolar copper, or at most 2millimolar copper. It will be evident that the solution initiallycomprises substantially more than 5 ppm copper; for example, the initialconcentration of copper ions or atoms can be at least 2 millimolar,e.g., at least 4 millimolar, at least 6 millimolar, or at least 8millimolar.

The resulting nanoparticles optionally have an average diameter betweenabout 1 nm and about 100 nm, e.g., between about 5 nm and about 100 nm,between about 10 nm and about 100 nm, between about 20 nm and about 50nm, or between about 20 nm and about 40 nm. Optionally, thenanoparticles have an average diameter of about 20 nm. The nanoparticlescan be of essentially any shape, but are typically irregularly shaped.

Essentially all of the features noted for the embodiments above apply tothese embodiments as well, as relevant; for example, with respect tonanostructure growth technique (e.g., VLS or VSS), type, composition,and size of the resulting nanostructures, ratio of nanostructures tosubstrate (e.g., silicon to graphite) by weight, incorporation into abattery slurry, battery anode, or battery, and/or the like.

FIG. 5 Panel A shows copper nanoparticles deposited on graphiteparticles by electroless deposition (row I) and adsorption (row II).FIG. 5 Panel B shows silicon nanowires grown from the nanoparticles;nanowire morphology and coverage on the graphite is similar for thedifferent deposition methods.

Nanostructure Growth Using Core-Shell Catalyst Materials

As noted above, gold (Au) nanoparticles have been extensively used forSi nanowire growth through the VLS mechanism. In this mechanism, asschematically illustrated in FIG. 4 Panel A, the Si from silanes in thevapor phase dissolves into mediating Au nanoparticles 401 deposited onsubstrate 400, to form Au—Si eutectic catalyst droplets 402. As theamount of Si in the Au—Si alloy droplet increases, the concentration ofSi eventually goes beyond saturation. The Si then evolves from liquiddroplet 403 to solid 404 and Si nanowires are formed. Solidified Aunanoparticles at the tips of the resulting nanowires are thereforetypical characteristics of VLS-grown nanowires. The diameter of the Sinanowires is determined by the diameter of the Au nanoparticles.

With increasing demand for silicon nanowires, the cost of goldnanoparticles becomes more significant, potentially limiting the use ofsilicon nanowires for applications such as batteries and medicaldevices. The cost of the gold nanoparticles is mostly based on thematerials price of gold. Methods described above focus on entirelyreplacing the gold catalyst with other materials. Another approach,however, is to decrease the amount of gold required.

Accordingly, one aspect of the present invention features methods ofproducing nanostructures (e.g., nanowires, e.g., silicon nanowires) inwhich nanoparticles that have a non-Au core encapsulated by an Au shellare used in place of solid Au nanoparticles as the catalyst particles.As schematically illustrated in FIG. 4 Panel B, catalyst particles 451deposited on substrate 450 comprise non-Au core 462 and Au shell 461.During VLS growth when a non-Au core is present in the system, theliquid-solid interface differs from that for a solid gold catalyst. Forease of explanation, it is assumed that the non-Au core does not reactwith Au and Si and thus the eutectic nature of Au—Si does not change.Thus, during the growth process, Si from silanes in the vapor phasedissolves into the Au from the shell to form Au—Si eutectic catalystdroplets 452 still containing the non-Au core 462. As the amount of Siin the Au—Si alloy droplet increases, the concentration of Si eventuallygoes beyond saturation. The Si then evolves from liquid droplet 453 tosolid 454 and Si nanowires are formed. Both gold and the non-gold corematerial are present at the tip of the resulting nanowire. For corematerials where reaction between the core material, Au, and Si doesoccur, phase diagrams of the new ternary gold alloys can be explored.

The Au atoms forming the shell react with Si atoms to form the eutecticalloy. Without limitation to any particular mechanism, the presence ofnon-Au alien species at the interface of the eutectic droplet and Siallows Si nanowires with comparable diameter to be grown from the coreshell catalyst particles as from single solid Au particles, whilerequiring less gold per nanowire. Thus, some percentage of Au can bereplaced by the non-Au component, reducing the cost of nanowiresynthesis. (Typically, the non-Au core material is much less expensivethan is Au.)

As one specific example of how much gold can be saved by employingnon-Au core/Au shell nanoparticles as catalysts for nanostructuresynthesis, FIG. 4 Panel C represents the percentage of the nanoparticlevolume occupied by the non-Au material (i.e., the volume of the core asa percentage of the overall volume including both the core and theshell) for a 15 nm non-Au core coated with an Au shell of varyingthickness. As can be seen from the graph, up to 82.4% of Au can be savedif 15 nm non-Au core particles covered with a 1 nm Au shell are used forSi nanowire growth. This percentage savings decreases as thicker Aushells are used.

As noted, the core of the nanoparticles comprises a material other thangold. Exemplary materials for the core include, but are not limited to:metal oxides, for example, Fe₂O₃, MFe₂O₄ (where M is, e.g., Fe, Mn, Mg,Co, or Ni) particularly spinel type, TiO₂, Al₂O₃, and ZnO; metals, forexample, Fe, Ni, Co, Pd, and Ag; and non-metal oxides, for example, SiO₂and other silicates such as polyhedral oligomeric silsesquioxanes. Thematerial constituting the core can, but need not, form an alloy withgold. In embodiments in which the material constituting the core formsan alloy with gold, the alloy can but need not exhibit a eutectic point.Silver, for example, forms an alloy with gold but there is no eutecticphase. In the example described above for silicon nanowire growth, Au isneeded at the surface to form an Au—Si alloy in the VLS growthmechanism. However, the core material can be designed to form an alloywith Au at one condition and at another condition not to form the alloy.In embodiments in which the formed alloy can also work as a catalyst forgrowth of other nanostructures such as nanotubes, nanobelts, and thelike, the processing conditions can be controlled to form chimericnanostructures between, e.g., silicon nanowires and othernanostructures.

The core is optionally between about 5 nm and about 500 nm in diameter,e.g., between about 10 nm and about 500 nm, between about 5 nm and about100 nm, between about 10 nm and about 100 nm, between about 5 nm andabout 40 nm, between about 5 nm and about 20 nm, or between about 5 nmand about 10 nm in diameter. The thickness of the gold shell isoptionally between about 1 nm and about 50 nm, e.g., between about 1 nmand about 40 nm, between about 1 nm and about 20 nm, between about 1 nmand about 10 nm, or between about 1 nm and about 5 nm.

In one class of embodiments, nanowires are grown from the non Au core-Aushell catalyst particles. Optionally, the nanowires are hollow. Suitablematerials for the nanostructures (e.g., nanowires) include, e.g.,silicon and/or germanium, as well as the inorganic conductive orsemiconductive materials noted hereinabove.

Essentially all of the features noted for the embodiments above apply tothese embodiments as well, as relevant; for example, with respect totype, composition, and size of the resulting nanostructures, type andcomposition of substrate (nonporous, porous, particles, graphiteparticles, sheets, wafers, etc.), ratio of nanostructures to substrate(e.g., silicon to graphite) by weight, incorporation into a batteryslurry, battery anode, or battery, and/or the like.

In a related aspect, nanoparticles that have an Au core encapsulated bya non-Au shell are used as catalyst particles for nanostructuresynthesis. Exemplary materials for the shell include, but are notlimited to: metal oxides, for example, Fe₂O₃, MFe₂O₄ (where M is, e.g.,Fe, Mn, Mg, Co, or Ni) particularly spinel type, TiO₂, Al₂O₃, and ZnO;metals, for example, Fe, Ni, Co, Pd, and Ag; and non-metal oxides, forexample, SiO₂ and other silicates such as polyhedral oligomericsilsesquioxanes. The material constituting the shell can, but need not,form an alloy with gold. In embodiments in which the materialconstituting the shell forms an alloy with gold, the alloy can but neednot exhibit a eutectic point.

Use of Au core-non Au shell catalyst particles, like use of non Aucore-Au shell catalyst particles described above, can be advantageous inreducing the amount of gold required for nanostructure synthesis. It canalso offer additional advantages, for example, by facilitating synthesisof core-shell nanostructures. For example, nanostructures having an Aucore and a Ni shell can be employed as catalyst particles. Core andshell sizes of the catalyst particles are selected to produce anappropriate composition for the alloy (e.g., 20% Au and 80% Ni), andnanostructure synthesis is performed under appropriate conditions withdifferent precursors (e.g., 20% silane and 80% ethylene) to producenanostructures (e.g., nanowires) having a silicon core and a carbonshell.

Essentially all of the features noted for the embodiments above apply tothese embodiments as well, as relevant; for example, with respect tosize of the core, thickness of the shell, type, composition, and size ofthe resulting nanostructures, type and composition of substrate(nonporous, porous, particles, graphite particles, sheets, wafers,etc.), ratio of nanostructures to substrate (e.g., silicon to graphite)by weight, incorporation into a battery slurry, battery anode, orbattery, and/or the like.

EXAMPLES

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims. Accordingly, the following examples areoffered to illustrate, but not to limit, the claimed invention.

Example 1 Synthesis of Colloidal Cu₂O, Deposition of Cu₂O Nanoparticleson Graphite Particles, and Growth of Silicon Nanowires From the Cu₂OCatalyst

Cu₂O colloidal nanoparticles are prepared as follows. Solution A isprepared by mixing 18 ml deionized water, 2 ml 0.1M copper sulfate, and30 ml 0.01M CTAB (cetrimonium bromide). Solution B is prepared by mixing48 ml deionized water, 1 ml 0.5M sodium ascorbate, and 1 ml 5M sodiumhydroxide. Solution B is added to solution A while stirring, and themixture turns to golden yellow almost instantaneously. The Cu₂O colloidsynthesized is stable for at least a few hours and has an averagediameter of about 45 nm based on light scattering measurements.

The Cu₂O colloidal nanoparticles are then deposited on graphitesubstrate particles as follows. In another beaker, about 10 g syntheticgraphite powder having an average diameter of about 10 μm is mixed with100 ml deionized water and agitated for at least 5 minutes using amagnetic stirring bar at a speed of 400 rpm. The graphite slurry is thenheated to about 65° C. under constant stirring. At about 65° C., all theCu₂O colloid synthesized above is slowly added to the graphite slurry,and the solution temperature is maintained at 60-65° C. for 30 minutes.Then the slurry is filtered under vacuum through a filtration apparatushaving a pore size of 0.2 μm and rinsed with copious amount of deionizedwater. The graphite cake is removed from the filter and then dried in anoven at 120° C. for at least 12 hours. The effluent collected in thefiltration apparatus is analyzed for copper concentration, which isabout 10 ppm. Cu₂O nanoparticles are adsorbed on the graphite particlesand the amount of copper is estimated to be 0.1% by weight (based on theamount of copper remaining in the effluent). See, e.g., FIG. 2 Panel A,which shows nanoparticles deposited on graphite particles underconditions similar to those described in this example.

Silicon nanowires are then synthesized from the Cu₂O catalyst particleson the graphite substrate as follows. 10 g graphite powder with the Cu₂Ocatalyst is loaded in a quartz cup with its bottom comprising a thinsheet of carbon paper and a disc of quartz frit, both of which arepermeable to gas but not to the graphite particles. The quartz cup iscovered with a quartz lid which is connected to the gas inlet port of ahot-wall CVD reactor. The reactor is ramped to a temperature of 460° C.,initially under vacuum and later in hydrogen flowing at 200 sccm orhigher. Silicon nanowires are grown at 45 Torr having a partial pressureof 1-4 Torr in silane, which is diluted by hydrogen and/or helium.

The graphite particles, after 45 minutes of growth, gain about 5-10%weight. The top layer of the graphite bed is lighter in color than thebottom layer due to gas depletion effect, which may be mitigated byflowing the reactant gases in both upward and downward directions oremploying a reaction system in which the graphite particles and thereactant gases are mixed more evenly throughout the growth process.Silicon nanowires grown on the graphite particles have an averagediameter between 10 nm and 100 nm and are essentially crystalline due tothe nature of the VSS growth mechanism. Some nanowires are straight butmost are kinked (see, e.g., FIG. 2 Panel B, which shows nanowires grownunder conditions similar to those described in this example). One end ofthe nanowire is attached to the graphite surface and the other end has aCu₃Si (copper silicide) tip.

Example 2 Electroless Deposition of Copper on Graphite Particles andGrowth of Silicon Nanowires From the Copper Catalyst

About 10 g synthetic graphite powder having an average diameter of about10 μm is mixed with 100 ml deionized water in a glass beaker andagitated for at least 5 minutes using a magnetic stirring bar at a speedof 400 rpm. 1 ml stock solution having 0.2M copper sulfate and 5.3Mformaldehyde is pipetted into the graphite slurry, and heated to about65° C. under constant stirring. At 60° C. or higher, 1 ml stock solutionhaving 0.4M Rochelle salt and 5M sodium hydroxide is added by pipet.After a 30 minute reaction at 60-65° C., the slurry is filtered undervacuum through a filtration apparatus having a pore size of 0.2 μm andrinsed with copious amount of deionized water. Optionally, the residualformaldehyde in the plating solution is treated with 0.6 g sodiumsulfite either before or after the filtration step. The graphite cake isremoved from the filter and then dried in an oven at 120° C. for atleast 12 hours. The effluent collected in the filtration apparatus isanalyzed for copper concentration, which is typically 1 ppm or below.The amount of copper catalyst in the graphite powder is estimated to be0.12% by weight (based on the amount of copper remaining in theeffluent). Copper nanoparticles vary in size between 10 nm to 100 nm andare distributed fairly evenly on the graphite particles. See, e.g., FIG.3 Panel A, which shows nanoparticles produced under conditions similarto those described in this example.

Silicon nanowires are grown from the copper nanoparticles on thegraphite particles under the same conditions as in Example 1, withsimilar results in weight gain and nanowire morphology. See, e.g., FIG.3 Panel B, which shows nanowires grown under conditions similar to thosedescribed in this example.

Example 3 Adsorption of Copper Complexes on Graphite Particles andGrowth of Silicon Nanowires From the Copper Catalyst

About 10 g synthetic graphite powder having an average diameter of about10 μm is mixed with 100 ml deionized water in a glass beaker andagitated for at least 5 minutes using a magnetic stirring bar at a speedof 400 rpm. 1 ml 0.2M copper sulfate solution is added by pipet to thegraphite slurry and heated to about 65° C. under constant stirring. At60° C. or higher, 1 ml stock solution having 0.4M Rochelle salt and 5Msodium hydroxide is added by pipet. After a 30 minute reaction at 60-65°C., the slurry is filtered under vacuum through a filtration apparatushaving a pore size of 0.2 μm and rinsed with copious amount of deionizedwater. The graphite cake is removed from the filter and then dried in anoven at 120° C. for at least 12 hours. The effluent collected in thefiltration apparatus is analyzed for copper concentration, which istypically 1-2 ppm or below. The amount of copper catalyst in thegraphite powder is estimated to be 0.12% by weight (based on the amountof copper remaining in the effluent). Copper nanoparticles vary in sizebetween 10 nm to 50 nm and are distributed fairly evenly on the graphiteparticles. See, e.g., FIG. 5 Panel A, row II, which shows nanoparticlesproduced under conditions similar to those described in this example.

Silicon nanowires are grown from the copper nanoparticles on thegraphite particles under the same conditions as in Example 1, withsimilar results in weight gain and nanowire morphology. See, e.g., FIG.5 Panel B, row II, which shows nanowires grown under conditions similarto those described in this example.

While the foregoing invention has been described in some detail forpurposes of clarity and understanding, it will be clear to one skilledin the art from a reading of this disclosure that various changes inform and detail can be made without departing from the true scope of theinvention. For example, all the techniques and apparatus described abovecan be used in various combinations. All publications, patents, patentapplications, and/or other documents cited in this application areincorporated by reference in their entirety for all purposes to the sameextent as if each individual publication, patent, patent application,and/or other document were individually indicated to be incorporated byreference for all purposes.

1-82. (canceled)
 83. A method for depositing Cu₂O nanoparticles on thesurface of a carbon-based porous substrate, the method comprising:providing a carbon-based porous substrate comprising a population ofparticles comprising at least one of natural graphite particles,synthetic graphite particles, graphene particles, carbon fibers, carbonnanostructures, carbon nanotubes, or carbon black; first mixingdeionized water with a copper source and a chelating agent to form afirst aqueous solution comprising copper ions, wherein the copper sourcecomprises at least one of copper sulfate, copper nitrate, copperchloride, or copper acetate and wherein the chelating agent comprises atleast one of Rochelle salt (i.e. potassium sodium tartrate), EDTA, orpolyols; second mixing deionized water with a reducing agent to form asecond aqueous solution, wherein the reducing agent comprises at leastone of sodium ascorbate, or ascorbic acid; mixing the first aqueoussolution with the second aqueous solution to form Cu₂O colloidalnanoparticles in an alkaline plating solution, the Cu₂O colloidalnanoparticles formed via chemical reduction of the copper source, andwherein the mixing comprises setting the pH of the alkaline platingsolution and setting the concentration of the copper ions in thealkaline plating solution to control the stability, the sizedistribution and the average size of the formed Cu₂O colloidalnanoparticles; immersing the carbon-based porous substrate into thealkaline plating solution comprising the formed Cu₂O colloidalnanoparticles; depositing the formed Cu₂O colloidal nanoparticles ontothe surface of the carbon-based porous substrate until the alkalineplating solution is substantially completely depleted of the copperions; and removing the carbon-based porous substrate with the Cu₂Onanoparticles deposited thereon from the plating solution and drying thecarbon-based porous substrate in an oven.
 84. The method of claim 83further comprising, prior to immersing, setting the copper ionsconcentration to at most 10 millimolar.
 85. The method of claim 84further comprising, prior to immersing, setting the pH of the alkalineplating solution between 8 and 11 and setting the copper ionsconcentration up to 5 millimolar.
 86. The method of claim 83, whereinthe particles in the population of particles in the carbon-based poroussubstrate have an average diameter between 0.5 μm and 50 μm.
 87. Themethod of claim 83, wherein the particles in the population of particlesin the carbon-based porous substrate have an average diameter between 2μm and 10 μm.
 88. The method of claim 83, wherein depositing furthercomprises depositing the formed Cu₂O colloidal nanoparticles onto thesurface of the carbon-based porous substrate until the concentration ofcopper ions in the alkaline plating solution is less than 1 ppm.
 89. Themethod of claim 83, wherein the formed Cu₂O nanoparticles in thealkaline plating solution have an average size between 5 nm and 100 nmas measured by a light scattering measurement or by electron microscopy.90. The method of claim 83, wherein the formed Cu₂O nanoparticles in thealkaline plating solution have an average size between 20 nm and 50 nmas measured by a light scattering measurement or by electron microscopy.91. The method of claim 83, wherein the formed Cu₂O nanoparticles in thealkaline plating solution have an average size between 20 nm and 40 nmas measured by a light scattering measurement or by electron microscopy.92. The method of claim 83, wherein removing further comprises filteringthe plating solution to recover the carbon-based porous substrate withthe Cu₂O nanoparticles deposited thereon and drying the carbon-basedporous substrate in an oven.
 93. A method for producing nanostructureson a carbon-based porous substrate, the method comprising: providing acarbon-based porous substrate comprising a population of particlescomprising at least one of natural graphite particles, syntheticgraphite particles, graphene particles, carbon fibers, carbonnanostructures, carbon nanotubes, or carbon black; first mixingdeionized water with a copper source and a chelating agent to form afirst aqueous solution comprising copper ions, wherein the copper sourcecomprises at least one of copper sulfate, copper nitrate, copperchloride, or copper acetate and wherein the chelating agent comprises atleast one of Rochelle salt (i.e. potassium sodium tartrate), EDTA, orpolyols; second mixing deionized water with a reducing agent to form asecond aqueous solution, wherein the reducing agent comprises at leastone of sodium ascorbate, or ascorbic acid; mixing the first aqueoussolution with the second aqueous solution to form Cu₂O colloidalnanoparticles in an alkaline plating solution, the Cu₂O colloidalnanoparticles formed via chemical reduction of the copper source, andwherein the mixing comprises setting the pH of the alkaline platingsolution and setting the concentration of the copper ions in thealkaline plating solution to control the stability, the sizedistribution and the average size of the formed Cu₂O colloidalnanoparticles; immersing the carbon-based porous substrate into thealkaline plating solution comprising the formed Cu2O colloidalnanoparticles; depositing the formed Cu₂O colloidal nanoparticles ontothe surface of the carbon-based porous substrate until the alkalineplating solution is substantially completed depleted of the copper ions;removing the carbon-based porous substrate with the Cu₂O nanoparticlesdeposited thereon from the plating solution and drying the carbon-basedporous substrate in an oven; loading the carbon-based porous substrateinto a reaction vessel wherein the population of particles with the Cu₂Onanoparticles deposited thereon form a packed bed in the reactionvessel; and growing nanostructures on the carbon-based porous substratein the reaction vessel from the Cu₂O nanoparticles via aVapor-Solid-Solid (VSS) synthesis technique, wherein the growingcomprises mixing the packed bed while flowing one or more reactant gasesin the reaction vessel during the nanostructure growing process; andwherein the nanostructures comprise silicon, germanium or a combinationthereof.
 94. The method of claim 93, wherein the nanostructures compriseat least one of nanowires or nanoparticles.
 95. The method of claim 93,wherein the one or more reactant gas comprise silane (SiH₄).
 96. Themethod of claim 93, further comprising, after growing, applying a carboncoating or an oxide coating to the nanostructures.
 97. The method ofclaim 93, further comprising, after growing, incorporating thecarbon-based porous substrate with the nanostructure grown thereon intoa battery slurry.
 98. The method of claim 93, further comprising, aftergrowing, incorporating the carbon-based porous substrate with thenanostructure grown thereon into a battery anode.
 99. The method ofclaim 93, further comprising, after growing, incorporating thecarbon-based porous substrate with the nanostructure grown thereon intoa battery.